Understanding H5 bird flu in New Zealand

Incubation periods of bird flu viruses

Avian influenza viruses have short incubation periods and current evidence indicates that birds begin shedding virus one to two days after infection, show clinical signs within two to five days, and may die within three to seven days if the infection is fatal. This means the effects of virus spread become visible quickly.

Understanding this short incubation period is important because it shapes how transmission chains develop. The arrival of an infected bird is only the first link. For the virus to establish itself, that bird must infect other susceptible birds, those birds must infect others, and this process must continue across multiple generations. If the chain breaks at any point, transmission stops. Many different disease introductions around the world never progress beyond the first few links.

H5N1 is not a virus that spreads silently. Because incubation is short, transmission chains—if they occur—develop rapidly. Birds infected by an initial case will begin shedding virus within days, and susceptible species may become visibly ill or die within a week. Sustained spread would therefore create detectable signals in monitored bird populations relatively quickly. It is unlikely that widespread transmission would remain hidden for long.

Its ability to cause obvious illness and mortality is precisely why it has had major impacts overseas and why New Zealand has invested heavily in preparedness. We ask for reports of three or more sick or dead birds, as they provide the clearest signals about what may be happening in wildlife populations and are important signals of potential transmission.

New Zealand has confirmed three birds with H5 bird flu since July 15 – two were single ocean-going birds and the other a native hawk. Our monitoring work through wildlife hospitals, public reports, and poultry operations has not detected any wider transmission. 

While absence can never be proven, the lack of additional detections means there is a low risk of any sustained transmission in bird populations right now, though we will continue to monitor closely.

Because the virus continues to circulate globally, it is reasonable to expect that infected birds will continue to arrive.

Assessing risk

We know the virus reached New Zealand because we detected it. We also know that, despite extensive follow‑up surveillance and the passage of multiple incubation periods, there is currently a low risk of ongoing transmission.

This does not mean the risk has passed. Infected birds will continue to arrive, and future introductions are likely to result in transmission. Birds migrate to New Zealand at different times of the year, and some will arrive infected, recover before arrival, or show signs of disease shortly after reaching the country.

New Zealand’s level of risk will change over time. It is influenced by the arrival of infected birds, global circulation patterns, and any evidence of local transmission.

A simple three‑level risk framework can help communicate periods of higher and lower risk across regions. This would allow wildlife managers and poultry owners to match their preparedness and biosecurity to the current level of risk. The goal is not to eliminate risk, but to recognise when it changes and respond appropriately.

For backyard and commercial poultry owners, elevated risk means strengthening biosecurity. Boots, vehicles, equipment, bedding, feed, and water must not become pathways for disease. Preventing contact between wild and domestic birds remains one of the most effective protective measures.

For wildlife, similar principles apply. Captive facilities may require enhanced biosecurity, while free‑living populations may benefit from temporary restrictions on access to sensitive areas. These measures reduce disturbance and limit opportunities for disease spread through human activity.

At present, the evidence suggests New Zealand has experienced introductions of H5N1 but there is a low risk of transmission. This distinction is important. It indicates that surveillance is working and that introductions can be detected. At the same time, future introductions remain likely, and vigilance will be essential to limit the effect on wildlife and poultry.

Monitoring bird flu in New Zealand

Our testing is focused on detection and tracking the movement of bird flu. Refer to our monitoring document for information on this monitoring, including the location of testing. We update it regularly.

Monitoring H5 bird flu [PDF, 1.5 MB]

H5 bird flu risk levels

Through surveillance and testing, we assess the current level of risk from bird flu. The current risk level for all of New Zealand is low.

H5 bird flu risk levels [PDF, 971 KB]