Former Conservation minister Chris Carter - who took a prominent role when the whale wars in the Southern Ocean went awry in 2007 - says Foreign Minister Murray McCully's "passive role" in the latest debacle is putting the lives of protesters at risk.

"The New Zealand Government is taking a passive role while Kiwi lives are put at risk and Japan's whalers get a green light to continue their whale hunt in the Southern Ocean," said Mr Carter, who is now Labour's foreign affairs spokesman.

He spoke as the New Zealand protest boat Ady Gil - the former record-setting earthrace trimaran - broke up and sank after being run down by a Japanese harpoon vessel.

The Sea Shepherd Conservation Society's 24m New Zealand powerboat had nearly 3m of its bow sheared off in a collision with the Japanese security ship Shonan Maru No. 2 - each side has since blamed the other for the ramming in which a New Zealand cameraman for the Animal Planet channel, Simeon Houtman, of Auckland, received broken ribs.

Mr Carter said the National-led Government had been "largely silent" while Japanese whalers continue to use dubious science as an excuse to continue to hunt whales in the Southern Ocean.

He said the Government did not bother to send a minister to the recent International Whaling Commission meeting in Portugal for the first time in 10 years.

"That absence was a clear signal that the New Zealand Government now sees whale conservation as a minor issue," Mr Carter said. Mr McCully's "muted response" to Japanese whaling would damage New Zealand's position as a leading conservation nation.

Government officials met with representatives from the Japanese embassy in Wellington to discuss the sinking, with Japan reported to have said it regarded the incident as "regrettable" but a "low-key event".

And Mr McCully later said Japan had not lodged a stern complaint with the New Zealand Government: "Japanese officials agreed with New Zealand that their citizens needed to have better regard for people on the high seas.

But he told Radio New Zealand that Japan has a responsibility to investigate the part its whaling ship played in the collision "and we will be pressing them to carry out that responsibility"

Australia's Environment Minister Peter Garrett said today Canberra may mount an international legal challenge to Japanese whaling if diplomatic negotiations with Tokyo fail to reach an outcome in the next 17 months.

"If we don't see substantial and significant achievement in respect of those negotiations, and if we don't see it by the time the International Whaling Commission meets in June of next year, then the consideration of legal action will be one that will be fully in front of us," Mr Garrett told reporters in Sydney.

Some legal experts believe the Japanese cull is in breach of international laws including the Antarctic Treaty System and the Convention on International Trade in Endangered Species.

Japan's government-backed whaling fleet aims to harpoon up to 935 minke whales and 50 fin whales, classified as endangered, in the Southern Ocean during the current hunt, "for research purposes".

The Japanese have accused the Ady Gil of suddenly slowing in front of the whaler, causing the collision.

The $2 million trimaran sank today as the anti-whaling protesters tried to tow it to safety at a French Antarctic research base, with the Sea Shepherd vessel Bob Barker reporting its last-known position to the Australian Rescue Coordination Centre at 8.20am (NZDT).

Crew members had been working around the clock in an attempt to save the NZ boat, and all fuel and lubricants had been removed.

The tow rope snapped after only 37km because the Ady Gil was filling with water, with the entire engine room and fuel tanks fully submerged.

A Dutch crew member on the Ady Gil, Laurens de Groot said its skipper, Aucklander Pete Bethune was very upset:

"He's devastated. This boat has been part of his life for the past five years. He's devastated".

Mr Bethune used the sleek, carbon-fibre trimaran was to set a record for circumnavigation of the world in 2008.

The Bob Barker's first officer Peter Hammarsedt later told the AFP newsagency:

"At this point unfortunately the Ady Gil is on the bottom of the Southern Ocean".