The Department of Conservation has commissioned testing on the corpses of dolphins washed up in the Hauraki Gulf.

The department said it was confident brodifacoum poisoning had been ruled out in relation to the deaths but was "conscious of the level of public concern surrounding this issue".

Brodifacoum poison was dropped on Rangitoto and Motutapu Islands to kill pests.

"As a result, we have commissioned further chemical tests on the dolphins, penguins and pilchard samples that will specifically look for brodifacoum poisoning," said DoC Auckland Area Manager, Brett Butland.

Testing has shown that dogs which died at Narrow Neck and Cheltenham beaches around the same time as the dolphins were found were killed by tetrodotoxin - a naturally occurring substance, found in tropical puffer fish, and in sea slugs taken from the beach.

The dolphin testing comes after marine biologist Karen Stockin, who is responsible for autopsies on common dolphins that wash up around Auckland, suggested a spike in the number of dead dolphins could point to a problem other than poisonous sea slugs.

Meanwhile, dead penguins were also reported in the Far North, Rodney, Auckland, Coromandel and Bay of Plenty regions, but penguin deaths, especially after storms, are relatively common for this time of year, says DoC.