Key Points:

An anonymous bird lover has put up a $1000 reward for information leading to the arrest of a suspected bird poisoner in Lower Hutt.

About 40 birds were found dead in Lower Hutt's Avalon Park on Friday morning, including gulls, starlings, pigeons and ducks.

An SPCA inspector took 10 survivors to a veterinary clinic, where two later died.

Six of the surviving eight birds have since been released into the wild.

Wellington SPCA spokeswoman Lisa Snow said an anonymous donor gave $1000 to the SPCA to use as a reward for information leading to the prosecution of the person or people responsible for the deaths.

"That was someone who was upset by what happened and was a great bird lover, so they wanted to help out as much as possible," Ms Snow said.

It was not yet known what caused the birds to become critically ill, but Hutt City Council was investigating, Ms Snow said.

She said it was unlikely to be a virus, because the deaths had happened across a number of species.

Last week's incident was the second case of mass bird poisoning within a month.

Nearly 80 birds were killed in Auckland's Western Springs Park on March 19 after a four kilogram box of grain laced with poison was released in the area.

Fifty birds were rescued and nursed back to health.

The incident was being investigated by police and Auckland City Council.