Cleaning boats: removing pests and water plants

Keeping your boat and related equipment clean helps stop the spread of aquatic pests around our coastline, and between our rivers and lakes. In some cases, it can even save you money.

Why clean?

If you have a boat, it's important to keep it clean – especially when you change location. If you don't, you could be spreading pests between New Zealand's lakes and rivers or around our coastline. Aquatic pests can affect:

  • aquatic habitats (including fish and shellfish habitats)
  • activities such as fishing, swimming, sailing and diving
  • aquaculture.

Regular cleaning also:

  • makes your boat and gear last longer
  • reduces vessel running costs because you'll use less fuel
  • reduces maintenance costs.

How to clean trailer boats

Before moving your boat to a new coastal area, lake or river:

  • wash the boat and trailer with fresh water if it has been in salt water, or a detergent solution if it has been in fresh water
  • remove fouling (like weeds, crabs, and barnacles) and dispose of appropriately on land – make sure to check the propeller, anchor well, anchor and chain
  • drain or thoroughly rinse areas where water pools
  • allow to air dry for several days before using it in the new location.

Check, clean and dry any equipment on your boat including mooring lines and buoys.

How to clean moored boats

Any boat that is moored in sea water (including yachts, launches, fishing boats and barges) can collect marine pests in the growth (biofouling) that builds up on the vessel bottom.
Before leaving for another part of New Zealand, check your boat for biofouling and clean it if necessary. If you're going for a while, we also recommend you reapply antifouling before you leave.

If you use antifouling paint on your hull, you should replace it:

  • once a year or as often as the paint manufacturer recommends
  • if the paint has been damaged
  • if your boat is getting a lot of fouling.

How to clean your boat hull

If you own a moored boat, the hull should never have more than a slime layer. We recommend you regularly brush or wipe off the hull – without damaging the antifouling.

You should also regularly clean your boat out of water and reapply antifouling paint. Watch our video explaining how to do this.

Video – How to clean your boat (7:52)

[The title "Clean boats living seas" appears with Ministry for Primary Industries logo. Camera pans to narrator speaking from a yacht in a marina]

Narrator: "As a boatie I love the great outdoors, and what we have in New Zealand is too precious to spoil with invading marine pests that don't belong here.

Peter [walking along marina]: "As the owner of a moored vessel you may not realise it but you have a vital role in ensuring marine pests are not transported around our coastline. And that is by keeping your boat clean and well antifouled."

Narrator: "Because it only takes one dirty boat bottom to give a hitchhiking fouling pest a ride to new locations where they can establish and wreak havoc."

[Peter sits down on yacht with narrator]

Narrator: "Now it's Peter's job to deal with pest species when they turn up in New Zealand waters, so he's seen some of the harm that they can cause. Peter, welcome aboard.

What's some of the damage these waterborne nasties can cause?"

Peter: "Well marine pests can really impact on some of the things we value in the marine environment. Things like going for a walk on the beach, taking a BBQ, boating into a pristine bay.

For example in Northland we have this sea squirt, it's come across from Australia at some stage.
[view of beach with large number of white sea squirts]

Over here it's a really ugly sausage-like thing, and it's all over the beaches of Northland.
[close-up of white, sausage-shaped sea squirts]

This particular beastie could only have got here on the bottom of a boat, and this is what it's doing to the beachfront at Paihia.

So another sea squirt that we've got here in New Zealand is Styela clava.
This is a sea squirt that's come from overseas somewhere, we're not sure where, but it's here in New Zealand.

[pale-brown sea squirts underwater and on shore]

It's had really devastating impacts on the Canadian aquaculture industry.

We don't know what the impacts are going to be in New Zealand, but it is here and it has been seen growing on things like scallops, like mussels, like oysters.

Another pest we have is Didemnum. It's in the Marlborough Sounds.

[underwater close-up of cream coloured organism covering a structure with shellfish]

It's really not pretty, it's covering all sorts of things, it's on marina pontoons, it's on the seabed, it's on aquaculture farms.

What it does is it grows over everything else, and smothers and kills it.

The jury's still out at the moment on what it might do in a natural environment, but it's not good.

There are a number of other marine pests around the world that we really don't want to get in New Zealand.

For example the Northern Pacific sea star,

[close up of 3 pink and purple coloured starfish attached to a rock]

it's a really devastating starfish.

Wherever it gets to it will just march through the area and eat everything in its path."

[large number of starfish are shown covering the sea bed]

Narrator: "Now New Zealand, as an island nation, is vulnerable to marine pests making their way here.

[map of Pacific showing shipping movements between New Zealand and other countries]

This graphic shows the huge volume of shipping movement around the Pacific over 5 years, and this doesn't include the many cruising yachts traveling the same area."

Peter: "While marine pest species can arrive in the ballast water of ocean-going ships, research has shown us that 2 thirds of species introduced to New Zealand have arrived on the bottom of vessels."

Narrator: "Biosecurity New Zealand (MPI) are working through a number of channels to prevent pest arrivals. But where they do establish, everyone has a part to play in ensuring that they don't spread... ruining your fishing, your diving, hey even your beach cricket, anywhere that you love the water."

[a yacht is being hoisted up out of the water in a marina]

Narrator: "As you've seen we've got some nasties here in New Zealand already, which can be easily transported to other bays and harbours by a dirty boat."

Peter: "That's right, and your part to play essentially involves good hull maintenance."

Narrator: "So Peter and I are going to show you the absolute best practice to ensure your boat not only goes faster and saves fuel, but protects New Zealand's marine environment."

Peter: "Generally, it pays to ensure your biofouling doesn't build up beyond a light slime layer."

Narrator: "In fact a good subtitle for this video could be 'keep your bottom clean'."

Peter: "Yep, because if you don't keep your boat clean not only do you become a biosecurity risk, but you're also increasing maintenance costs."

[person runs hand over boat hull in marina, removing marine growth]

Narrator: "Start with checking for larger growth on parts of your boat that have been beneath the waterline. It may be that in checking what's there you come across something unusual, something you've never seen before. If this is the case, remove the sample, plastic bag it, and put it aside to freeze.

[man removes a marine organism from hull with a metal scraper and places in a plastic bag. Pest reporting hotline number appears: 0800 80 99 66]

Most likely it will be business as usual, so begin cleaning by hand, removing any of the larger more visible fouling."

Peter: "And it's really important to dispose of that fouling in the bin where it can't get back into the ocean and cause more trouble and reproduce."

Narrator: "You'll need to have an especially good look around what we call niche areas – places that stick out or contain water where marine pests could attach or hide. This includes the prop, water intakes, rudders and casings, and live bait tanks. To thoroughly clean your boat, at a minimum hose and brush down all surfaces or water blast to remove all traces of fouling and slime.

[boat hull and niche areas being washed down with water blaster]

"Ideally you should hose down or water blast in a facility or yard that has systems in place to contain waste runoff and prevent its washing back into the sea. Once again with your washing down be really thorough around those niche areas. They're prime locations for harbouring pests."

Peter: "In order to effectively keep marine pests off your boat, it's really important to clean and antifoul according to the manufacturer's instructions."

Narrator [standing next to tins of antifouling paint in a shop]: "The length of time you can get out of your antifouling depends on many factors including how well it was applied in the first place, the type of antifouling you use, how much you use your boat, and the kind of boating you do.

Now obviously you can refer to your paint retailer for advice on this. Antifouling paints generally only last between 1 and 2 years.

And you may have heard the handy tip about using a different colour antifoul each time. This is so you're clearly able to see when you're due for a repaint.

Right, the basic tips about repainting.

[3 men are sanding a boat hull with sand paper] Start by wet sanding your existing antifoul with 80 grit or something similar.

"Ensure the hull is clean and dry before painting. Areas where your antifouling has worn back to the hull surface, or has been damaged, will first require painting with a marine primer.

[small patch on boat hull is being hand painted with primer]

"When the primer paint's dried to a soft but non-tacky feel – this means you can leave a thumbprint but not get any paint on your thumb - then it's time to apply the antifouling paint.

[antifouling paint is sprayed on boat hull with a paint gun]

"Apply a good coat and ensure you don't miss those niche areas. Unless specifically requested by the manufacturer, do not thin antifoul unless absolutely necessary for application. This will compromise its performance. Your retailer will be able to advise on the quantity required for optimum performance for your type of boat.

Antifouling is most effective with several coats applied, and the number of coats applied generally corresponds to the number of years of protection you can expect. It's important to allow each coat to dry between applications, which could take anything from 4 to 24 hours depending on the weather.

Remember we're talking here about best practice, so while some of you may clean and antifoul on tidal grids, to get the best performance and value out of your antifoul, it's highly recommended you apply it on the hard stand.

An area that can cause a problem with pest attachment is the parts of the hull that have been beneath supports drops or pads during painting. So obviously when you're applying subsequent coats of paint try to move the supports to ensure the entire hull gets an even treatment.

For the best results from your antifoul, leave it to cure for 24 hours before relaunching your boat.

[Narrator talks from yacht in water as Peter unties it and pushes it away from marina]

"Well that's biosecurity hull maintenance 101."

Peter: "Yep, it's not comprehensive but there is more information on the Biosecurity New Zealand (MPI) website."

Narrator: "But by following this pretty simple process not only will your boat be more efficient, save on fuel, and go faster, but you'll also help protect our seas… and let's face it – our enviable outdoor lifestyle, for generations to come."

[End of transcript]

New Zealand's Clean Boating Programme has instructions for cleaning moored boats in their Clean boating guide.

Pay particular attention to:

  • the hull, keels and stabilisers
  • intakes and outlets
  • propellers and shafts
  • rudders, rudder shafts and casings, rudder recesses
  • anchors, anchor chains and anchor wells.
Side view of 2 boats showing areas to clean
Areas to clean on your boat

How to apply antifouling

Use an antifouling paint that's suitable for your boat type and its use.

For best results, clean, sand and prime the hull before you apply 2 or more good coats of antifouling paint. Let the antifouling dry completely between coats and, ideally, leave your boat out of water for 24 hours after the last coat.

Where to clean your boat hull

Most New Zealand marinas have haul out facilities and hardstands. Contact the marina to find out about their cleaning facilities.

Find out more

Who to contact

If you have questions about cleaning boats, email

If you think you've found a marine or freshwater pest, call our pest-and-disease hotline on 0800 80 99 66.

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