Independent laboratory tests on dolphins that died recently in the Hauraki Gulf have come back negative for both brodifacoum (rat poison) and tetrodotoxin - the toxin found in sea slugs that was responsible for recent dog deaths.

The tests - on dolphin livers and stomach contents - were commissioned by the Department of Conservation and carried out by independent laboratories in Nelson and Wellington.

The tests confirm earlier findings by veterinary scientists and pathologists ruling out any link between dog and other marine life deaths and the recent pest eradication operation involving brodifacoum on Rangitoto and Motutapu Islands.

"We commissioned the tests to try to find out how the dolphins died, but also to reassure the public that the dolphin deaths were not related to the recent Rangitoto-Motutapu restoration project," says Auckland Area Manager Brett Butland.

The cause of the dolphin deaths is still unknown and DOC has asked the Cawthron Institute to test the dolphin samples for domoic acid - an algal toxin known to be a common cause of dolphin deaths. Results are expected this week.

Tests on pilchards (taken from dead dolphins' stomachs), mussels and pipis have also come back negative for brodifacoum.

DOC also commissioned Landcare to carry out toxicology tests on five blue penguins. Low level traces of brodifacoum were found in the livers of two of the penguins but this has been ruled out as the cause of death.

Further test results from the New Zealand Centre for Conservation Medicine (NZCCM) indicate that starvation is the likely cause of death of penguins from the region. This supports earlier findings by NZCCM.

Penguin mortalities are not uncommon for this time of year, and have been reported in the Far North, Rodney, Auckland, Coromandel and Bay of Plenty regions.