A Ministry report obtained by Newshub has revealed extensive illegal activity in the set net fishery off Kaikoura, including illegal dumping, misreporting and "virtually non-existent" reporting of marine mammal bycatch.

The Kaikoura coast is home to a host of marine life and a popular set net fishery. The Operation Loctite report, which details illegal behaviour by four out of five set net fishermen in the area, was leaked to Newshub.

Even new Fisheries Minister Stuart Nash wasn't aware of it.

A compliance team investigated in 2011 to see if fishers were obeying the 24-hour soak time rules - that's the time limit for how long a net is allowed to be left in the water before being collected.

The team uncovered a range of illegal activity, with fishers leaving nets in the water for seven times the limit in some cases.

A dead fur seal was hauled up in one net and quote species of fish were dumped. False reporting of catch was noted, and reporting of marine mammal catch was "virtually non-existent".

Minister Nash called the activity "totally unacceptable".

"There are rules in place for a very, very good reason," he said.

Yet in the case of Operation Loctite, no one was prosecuted due to a loophole in the law.

Gary Orr, Ministry of Primary Industries (MPI) manager for Compliance and Investigations, says things have improved since 2011.

"There have been changes made in the regulations since, and we've worked closely with fishers to improve compliance."

Yesterday, the Minister revealed five Hector's dolphins were caught in a single net - information which only came to light after the skipper notified authorities.

MPI accepts this type of thing can go unreported.

"Quite possibly, yes," Mr Orr said in response. "We would be naive to think otherwise."

The Loctite report states that when one skipper was busted, he claimed he was unaware of the soak time rules even though he'd been caught before.

Warning letters issued by officers were "met with defiance".

Executive director of Greenpeace Russell Norman says the report is symptomatic of an industry-wide problem.

"It tells you that the fishing industry has become accustomed to routinely breaking the rules and getting away with it."

Forest & Bird's Geoff Keey says changes must be made.

"They clearly considered themselves above the law," he told Newshub.

"It also shows why we need cameras on boats and why the set net fishery has had its day. It needs to go."

Two of the skippers who were warned after the 2011 report were again caught in 2015 breaching soak time rules. MPI points out on that occasion they did prosecute, and the skippers were fined $3000 each.

Anyone with more information can contact Michael Morrah at michaelmorrah@mediaworks.co.nz.

Newshub.