New Zealand is in danger of losing its status as a world leader in managing fisheries, says the researcher behind a new documentary on overfishing.

Charles Clover, a former environment editor at Britain's Daily Telegraph newspaper, says New Zealand's reputation is under threat from its orange roughy catch and an almost complete lack of knowledge about the health of many of its fisheries.

A documentary film based on Clover's book The End of the Line opens in New Zealand tomorrow after an influential run in Britain.

The British food-shop chain Pret a Manger ditched tuna sandwiches and several high-end restaurants changed their fish-buying policies after it screened.

In the film, scientists warn that fish stocks will be in dire trouble by the middle of the century if current rates of fishing continue.

They claim that modern, high-tech fishing fleets with their mammoth nets give fish virtually no chance, and that catch limits are routinely flouted by some unscrupulous crews. Fishing boats have the capacity to catch the world's fish four times over in a year, they say.

Clover, a keen recreational fisherman, wants shoppers to check the source before they buy fish and pressure Governments to police the seas as strongly as the land.

He said New Zealand was held up as a beacon of fisheries management, with Iceland, but "the closer you look at it, the more holes there are in it".

He quoted information from the Ministry of Fisheries showing that last year, officials had enough information to report on the health of 117 fish stocks or substocks out of 628 managed under New Zealand's Quota Management System.

"The number one hole in your system [is that] you don't actually have a proper up-to-date assessment of the 600-odd species you have in your waters," Clover said.

"Of the assessed stocks, 32 per cent are defined as depleted or collapsed, which is more than in America."

Clover said a policy of catching orange roughy - a long-lived deep-sea fish boycotted by some British supermarkets - was considered outrageous in Britain. The Ministry of Fisheries says it is managing orange roughy by restricting catch limits where needed to save the stock from extinction.

But the fish - and New Zealand hoki - has been blacklisted by Waitrose supermarkets because it is caught by destructive bottom trawling.

Despite its faults, Clover said New Zealand's fisheries management system was one of the best.

"The world has got pretty universally inadequate fisheries management systems, so yours is better than most."

New Zealand's network of marine reserves, once thought to be a world leader, was in danger of being overtaken by Europe and the western United States, he said.

Under the Biodiversity Strategy, New Zealand was to triple protected areas to 10 per cent of its marine environment by this year by shielding rare and outstanding areas, as well as a representative slice of each ecosystem.

But the designation of new areas for protection has been halted after Department of Conservation funding cuts.

Figures from Statistics New Zealand released on Wednesday show the value of New Zealand's commercial fishery rose to $4 billion last year, 20 per cent of that from hoki.