Dead fish, most of them snapper, have washed up on Coromandel Peninsula beaches, and fisheries authorities want to know how they got there.

The rotting carcasses are littering beaches from the top of the Coromandel Peninsula for about a 10km stretch, Newstalk ZB reported.

Fisheries authorities want to hear from anyone who might have a clue as to what happened.

In similar cases in the past, prosecutions have resulted against commercial trawlers dumping their catch at sea.

Compliance manager Brendon Mikkelsen says the fish started washing up on New Year's Day.

"There is a significant volume of snapper and we're currently investigating it. The positive is that we have some witnesses coming forward so we're speaking to those witnesses."

He is asking for public help to shed light on how the fish got there.

"We're most interested in information from anybody that's seen any activity at sea that's associated with that fish, so obviously vessels in the area."

Mr Mikkelsen warned people not to eat the fish as there was no telling how long they had been floating at sea in the sun.

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Dead fish wash up on Coromandel beaches