A marine ecologist is investigating whether there is a link between a high number of dolphin deaths in the Hauraki Gulf and the deaths of other sea life and dogs around Auckland.
Auckland Regional Public Health Service is warning the public, particularly children, to stay off the eastern beaches while authorities find the cause of the problem.
At least two dogs have died and several others have become ill.
Massey University Marine Ecologist Karen Stockin says seven dolphins have died in a three-week period, an unprecedented number for this time of year.
She is examining the dolphins' bodies to find the cause of their deaths, but says it is too early to confirm whether they died from the same thing that is killing other sea life and dogs.
Biosecurity officials say the mystery toxin does not appear to pose a significant biosecurity risk.
MAF Biosecurity senior animals response advisor Naya Brangenberg says the dogs' symptoms suggest they came into contact with toxic algae.
She says officials may never find what is causing the deaths because there are so many possibilities. More tests are expected next week.