Japan looks set to be called to account over accusations that it has grossly overfished southern bluefin tuna stocks in southern waters including the Tasman Sea.
The tuna species, valuable for its high fat content, came under voluntary conservation measures in the mid-1980s after heavy fishing resulted in a significant decline in mature fish.
The Commission for the Conservation of Southern Bluefin Tuna, of which New Zealand was a founding member with Australia and Japan, was established in 1993 to manage the stocks.
The Minister of Fisheries, Jim Anderton, said yesterday that reviews of market data in Japan allegedly showed a huge discrepancy between what was being sold and what was being caught.
"They are selling fish they have theoretically not caught".
The independent findings will be analysed by a scientific committee, due to meet in October, which will advise the commission.
Mr Anderton said New Zealand was deeply concerned about the vulnerable status of the stock and would be very disappointed if the global quota had been substantially over-caught.
He said while the figures were to be confirmed, it appeared that the Japan over-catch was "very, very significant" and could see the country penalised in the next quota allocation.
He admitted New Zealand had on occasion over-caught a "modest amount" of bluefin tuna.
"But we have always paid this back in subsequent years through under-catching by the same amounts. I expect that in this instance, if there is any substantial over-catch, this would be paid back by the nations involved, and that the commission would take action to prevent this occurring again."
Mr Anderton said the management system relied on the integrity of the countries which had signed the agreement to honour the conservation convention.
He understood Australia had also breached its quota, but not by a huge amount.
The Australian fishery nets the juvenile tuna as they pass through its southern waters and tows them close to the mainland where they are placed in floating cages anchored to the ocean floor.
The tuna are then fattened for several months and sold directly to Japanese markets as frozen or chilled fish for the sashimi market.
The main method for other countries is longline fishing, with the fish shipped frozen to markets in Japan.
Jane Willing, Ministry of Fisheries manager, international fisheries, said Japan caught most of its southern bluefin tuna from the high seas off South Africa, and then in the Tasman Sea.
Ms Willing said the species, which spawned near Indonesia in the Indian Ocean, were highly migratory and by the time they reached New Zealand waters were quite large.
They were fished here under the quota system mainly off Tauranga or the West Coast.
* Japan: 6065 tonnes
* Australia: 5265 tonnes
* Korea: 1140 tonnes
* New Zealand: 420 tonnes