Up to 40 tonnes of dead blue mackerel could be sloshing around in Auckland's Hauraki Gulf, a coastal fisheries expert says.

Locals have reported cleaning trailerloads of fish off the shores of Kawau Island, while numerous reports of dead fish have been received by the Ministry for Primary Industries this week.

Posters on social media said birds weren't feeding on the fish - apart from pecking out their eyes.

Fingers have been pointed at fishing trawlers with some assuming the fish - reportedly all blue mackerel - were deliberately dumped.

But the Ministry for Primary Industries now believes it knows the source of the dead fish.

"We have a report regarding the accidental loss of 1,000kgs of mackerel from a net approx. 5 nautical miles from Kawau Island," an MPI spokeswoman said.

"This leads us to believe that it is highly likely the fish around Kawau are those fish that were accidentally lost from the commercial fishing vessel."

Dead mackerel in the water surrounding Kawau Island in the Hauraki Gulf. Photo / Supplied

Scott Macindoe, spokesperson for non-profit LegaSea, earlier told Newstalk ZB that the huge number of dead fish made him think a natural event was to blame.

Thousands of fish had been found around Kawau, he said. LegaSea had picked up more than 100 fish off the northern beach and taken them out to sea; at the southern end they were 2-3 deep in South Cove and Bostaquet Bay.

 

Heading to Aotea/Great Barrier Island on Saturday he found a big patch about 8 miles east of Kawau, and more that evening off Sandy Bay and Fairchild's Reef.

Macindoe believed the event had killed the fish on Tuesday or Wednesday as they were "pretty smelly" by the time they arrived at Kawau.

Blue mackerel weren't often seen in New Zealand shops - they were exported whole, frozen, for about $2/kilo.

"We export eight million kilos of blue mackerel per annum at what I think are silly prices, low prices - so this is a drop in the bucket really."

Dead mackerel lining the shores of Kawau Island. Photo / File

Ministry for Primary Industries fisheries compliance manager Steve Ham said the compliance team had been made aware of the issue by multiple people.

MPI was making enquiries with commercial fishers and companies that had vessels in the area around the time, he said.

"Samples of the fish had also been taken for testing to rule out any natural event that may have caused the deaths."