The Auckland SPCA says a "massive" outbreak of botulism is killing hundreds of birds.
The charity's chief inspector, Vicki Border, says she has not seen this scale of fatalities in her 10 years at the SPCA.
Birds that are still alive but have contracted the illness are basically paralysed, she says.
The disease thrives in shallow, warm water and Ms Border says warmer temperatures at present are helping the bacteria which produces the toxin to multiply.
Members of the public are advised to call their local SPCA branch if they see any sick birds.