Auckland Council is partly blamed for an outbreak of botulism, a disease that kills water birds.
The disease paralyses birds causing them to either drown or starve to death.
The Bird Rescue Trust says that while hot weather often encourages botulism, the council also is failing to maintain its waterways.
Trustee Hillary Stollery says putting barley straw in ponds to purify the water would halve the number of botulism cases.
The council says it already does that and its contractors regularly monitor stormwater ponds.
The SPCA says warmer temperatures are heating ponds and helping the bacteria which produces the toxin, to multiply.