A commercial dump of gurnard 35 kilometres west of Piha beach through to Karekare beach

One of New Zealand's largest fishing companies has defended its loss of juvenile gurnard into the Tasman Sea off Auckland this week saying there was only a "leakage" of between 40 and 45 kilograms of fish.

But the man who went on social media to expose what he said was a major fish dumping says there was much more than that.

The Ministry for Primary Industries (MPI) says it accepts the loss was accidental and will take no further action.

Coromandel based trawler Eight Bells, operating for Moana Pacific Fisheries, a subsidiary of the largest Maori-owned Aotearoa Fisheries (AFL), was behind the incident which recreational fisherman Kerren Packer filmed when he found dumped young fish off Piha and running into Karekare Beach.

Eight Bells owners have not returned requests for comment.

AFL head Carl Carrington said they do not condone the dumping of fish as part of commercial fishing practice.

"We are vehemently opposed to any activity involving fish dumping at sea by commercial operations," he said.

Dumping went against the purpose of the quota management system and his company was entirely committed to the sustainability of New Zealand's fish stock, he said.

There were many operators fishing in the area, a number specially targeting gurnard.

"Our boat in the area at the time had no need to dump any fish – it has sufficient quota, and anything it lost was income lost to that operator."

Carrington said they were confident of the MPI's description of the incident as an accident, adding that a degree of accidental loss was unavoidable in commercial fishing.

"Commercial operations naturally encounter some leakage from around the sides of a catch, due to nets having holes" he said.

Their boat in the Piha area over this time reported accidental loss in the vicinity of 40-45 kilograms over two days – less than half a percent of the total haul.

"Typically we see leakage of less than one per cent of a total haul; we're talking about a small number."

MPI official Dave Turner said the incident was a natural part of commercial fishing.

"They had a big bag of gurnard – extremely large – and as they were hauling, they just fell into the sea," he said.

"Accidental loss happens on occasions; there might be rough water, big seas, they might have trouble controlling the gear. That might cause accidental loss.

"They reported as per requirement, the forms have been filled out."

Packer said when he came across the fish there was less than a half metre swell and there was much more than 45 kilogrammes of fish in the water - "untold more kilograms," he said

Although he had complained about the dumping, MPI never investigated properly and had not spoken with him.

Their investigation had been "roughshod and Yes Minister," he said.

"What this needs is a proper formal investigation."

Labour's fisheries spokesman Rino Tirikatene said he was pleased AFL had fronted up but said it was a member of the public who spotted it.

"This is a real wake up call for all fishers to ensure they follow the law," he said.

Maori values of kaitiakitanga or guardianship of fisheries was fundamental to Maori.

"AFL owes its existence to the Maori commercial fisheries Treaty settlement. AFL must be vigilant and ensure its activities respect the treaty rights and values from which it comes."