Five dolphins are being tested in connection with the mystery deaths of dogs and sea life.

However, researchers believe they may be dealing with two or even three unrelated causes of illness in the Hauraki Gulf.

Massey University marine biologist Karen Stockin said seven common dolphins had died in the gulf in the past month - an unusually high number for this time of year - as well as blue penguins and hundreds of pilchards.

Two of the dolphins drowned after becoming tangled in nets but another four deaths were being investigated.

A seventh dolphin was found yesterday.

Large numbers of pilchards died in 1995 and 1998 from a virus that attacked their gills, indirectly killing little blue penguins because not enough pilchards were available for them to eat. Dolphins were never tested for pilchard-related effects.

But Dr Stockin said a number of the latest dolphins died with stomachs "absolutely packed full" of pilchards - ruling out starvation but not another problem related to the fish.

"At the moment ... we've got a slightly higher-than-usual incidence of little blue penguins dying, we've got an elevated incidence of dolphins dying ... and on top of that we've got a rather large, bad die-off of pilchards. It is quite possible that the pilchards and the little blue penguins are connected for a malnutrition issue but it's nothing to do with the dolphins, or it's possible the dolphins are involved but it doesn't involve the dogs," Dr Stockin said.

"We might even have three different scenarios going on."

The Ministry of Agriculture and Forestry is treating the dog deaths as unrelated to pilchards and has warned that the mystery of ill dogs may never be solved.

Tests for toxic algae, which can be fatal to dogs, came back negative in tests on vomit from the two that have died.

Niwa algae expert Dr Hoe Chang said it was likely people would have seen the water change colour if algae were the cause.

But a MAF spokeswoman said the ministry was not ruling it out. Toxic algae killed thousand of fish in the Hauraki Gulf in 2002.

Pilchards tested so far have been too rotten to examine for the gill-attacking virus, which breaks down quickly after the pilchard dies.

People who find large numbers of dead or dying fish should phone MAF on 0800-80-99-66.

* The theories

Toxic algae: Stomach contents came back negative but authorities are not ruling it out.

1080/Brodifacoum rat poison: Dog symptoms don't match.

Pilchard virus: Could be responsible for pilchard deaths, penguin deaths and possibly others but no reliable testing for the virus yet.

Seaweed: Is being tested from the beach near where the dogs died.

Deliberate poisoning: Has happened before but authorities believe a natural toxin is more likely.