People are being urged to keep away from dead birds in or near waterways in Kapiti.
Signs of avian botulism, a disease causing paralysis and death in birds, have been identified in ducks and seagulls at Waikanae Beach's Pharazyn Reserve.
"Birds affected with avian botulism typically show signs of paralysis, are unable to fly, and have drooping heads," council biodiversity programme manager Rob Cross said.
"Wild and captive bird populations, most notably waterfowl, can contract the disease, but it does not cross over to human beings."
Cross said the disease is spread to other birds through the carcasses of those killed by avian botulism, and can make dogs and cats sick if they eat a dead bird infected with the disease.
"Anyone who sees a dead bird in the reserve area is strongly advised not to touch it — they should inform the council so our staff can remove them and dispose of the carcasses safely."
Health authorities say avian botulism is unlikely to pose a threat to humans, but can be passed on to cats and dogs.
Cross said pet owners should be vigilant and ensure pets don't go near any bird carcasses.
Most outbreaks of avian botulism occur during summer when there is less oxygen in waterways due to lower water levels.
This causes decaying plant growth and the perfect microclimate for the toxin to occur.
Outbreaks are managed through the removal of infected carcasses and, once cooler weather comes, the disease generally runs its course.
Anyone who sees sick birds at council reserves, as opposed to those that have already died, should inform the SPCA.