New Zealand is set to veto any attempt to completely protect the world's last unexploited ocean - so a lucrative fishing industry can continue operating.

An official New Zealand document leaked to Fairfax reveals Wellington, backed by the US, does not want the whole 650,000 km2 Ross Sea declared a marine protected area (MPA), despite a 25-nation convention saying it is "of high global importance".

Maps in the document written by the Ministry of Fisheries show a large area of the Ross Sea is excluded from a marine park. It means the fishing industry can keep taking toothfish, discovered by New Zealand in 1996, worth $18 million a year.

The world's southern-most fishery is 800km north east of Scott Base.

The paper says toothfish are "an ecologically important species", but many more fishing boats may chase it.

The move will shock about 500 international marine scientists behind The Last Ocean Project, a US-based campaign that won international attention during the saga of Happy Feet, the emperor penguin.

New Zealand aims to protect 75 per cent of the emperor and adelie penguins.

Wellington will present the plan to the Commission for the Conservation of Antarctic Marine Living Resources.

The leaked paper reveals its detail and compares it with a US Government counter-proposal.

"The New Zealand scenario covers less of the historical fishing grounds for Antarctic toothfish than the US scenario and avoids those areas in which historical fishing effort has been concentrated," the paper says.

Sources involved said it had been hoped the US would designated the entire Ross Sea an MPA, but the US State Department deferred that by way of "honouring New Zealand ownership of the Ross Sea".

The US would have included "much more critical habitat for full functioning of the Ross Sea ecosystem".

Toothfish, a late-maturing, slow-growing, long-lived species that can grow up two metres long, fetches about US$70 (NZ$95) a kilogram. The US takes about 80 per cent of New Zealand's catch, much of it ending up in the gambling city of Las Vegas.

Because of its high retail cost, toothfish - sold as "Chilean sea bass" - is not available in shops here.

The document suggests environmental risk by New Zealand by picking where best to catch the fish.

"Scientists judged that displacing catch away from juvenile and pre-recruit fish in favour of catching adult fish may serve to minimise the risk of ecosystem effects of fishing as well as helping to deliver fishery outcomes and providing opportunities for science," the paper says.

It says "it was thought" that protecting some of the adult feeding area "may provide some benefit by helping to maintain the full range of size classes of larger fish, to counter potential genetic selectivity effects of the fishery on growth and maturity over time".

Termed an "Olympic fishery" - those with quota have to fish in the limited season regardless of the weather conditions - the paper says New Zealand wants a large enough area "to accommodate the expected number of vessels without forcing vessels into areas where health and safety will be compromised".

Earlier this year influential American biologist David Ainley warned New Zealand's approach and industrial fishing threatened permanent change.

"Over a million of these large predatory fish are now gone, along with the 1000 year old sponges ripped up in the long-lining process. An increasing group of scientists feel this fishing has to stop."

With sharks largely absent from the Ross Sea, toothfish, a massive carnivorous, play a key role in the ecosystem.