Avian botulism is thought to have killed around 80 birds in the Piako River in Waikato and more than 150 are displaying symptoms of the paralysing disease.
Gulls, black shags, grey teal, mallard ducks, grey ducks, pied stilts, white-faced herons and royal spoonbills are some of the species so far claimed by the disease.
Auckland Waikato Fish & Game has planned an operation to stem the outbreak near the river's outlet into the Firth of Thames.
The botulism is a form of food poisoning caused in water birds by hot weather releasing toxins from dead weeds or other pollutants in shallow, still water.
The toxin causes birds to lose control of their head and necks before the paralysis spreads to the rest of their bodies.
Fish & Game southern gamebird manager David Klee said the symptoms exhibited by birds in the area were consistent with the disease.
He said it was impossible to accurately establish how many birds have died because the site was subject to large tidal movements which may have washed dead or dying birds out into the Firth of Thames, beyond the range of Fish & Game's monitoring.
"We have received reports of birds washing up along the Thames coast and fishermen have reported seeing carcasses out at sea."
An operation, also involving the Department of Conservation, would be carried out tomorrow to try to halt the spread of the disease.
"It is possible to manage outbreaks of avian botulism by removing dead and dying birds and disposing of them in an appropriate manner," Mr Klee said.
"However, collecting a large number of carcasses over a wide area requires a lot of manpower. That's why we've rallied our game bird hunting licence holders and are calling on other volunteers to help out."
Anyone interested in helping can contact Mr Klee on (07) 849 1666 or 021 300 183 (after hours) or email email@example.com.