Unusually large numbers of dead pilchards have mysteriously been washing up on the beaches north of Auckland.

The disclosure from Biosecurity New Zealand comes in the wake in the wake of reports that a mystery illness has affected a large numbers of dogs, and killed two, following visits to North Shore City beaches.

The Ministry said that it had investigated several incidents of sick dogs, including four from Narrowneck Beach, one from Browns Bay and one from Waiheke Island.

The fish have been washing up on Whangaparaoa Peninsula and Martins Bay and have been sent for testing to rule out a disease with results due back next week.

"It is too soon to tell if there is any connection between the dead fish and the dog illness, but given that they occurred in different locations, they are being treated as unrelated at this time."

The Ministry of Agriculture and Forestry's biodiversity division said it had received a large number of reports in recent weeks about dogs suffering from repeated vomiting. While most have recovered quickly, it said some had developed seizures and died.

There was no indication so far as to what had caused the illnesses or deaths but pathology results from the dog that died yesterday are due back this week, MAF said.

"There has been nothing so far to indicate the situation is due to any form of chemical or pesticide poisoning."

Anyone who found unusual numbers of dead fish or other marine life is urged to report this to the Ministry's pest and disease hotline: 0800 80 99 66.