The crew of the New Zealand trimaran harassing Japanese whalers in the Southern Ocean had to be rescued after their boat was rammed and sunk by a Japanese ship, anti-whaling group Sea Shepherd Conservation Society says.



The confrontation is thought to have happened early today in the area of Commonwealth Bay off the Adelie Coast of Antarctica.



The former Earthrace boat - now known as the Ady Gil - was captained by Aucklander Pete Bethune and had four other New Zealanders and a Dutchman crewing it.



Mr Bethune said before his departure he would not follow previous Sea Shepherd tactics and try to ram Japanese whalers.



Sea Shepherd leader Paul Watson told theage.com.au the Ady Gill had been cut in half by the Japanese whaler acting as a security vessel for the Japanese fleet.



The $1.5 million high-tech vessel's remains were sinking, but its six-man crew had been rescued and was uninjured, Mr Watson told The Age. He said it was idling in waters near Commonwealth Bay when it was suddenly approached and rammed by the Japanese ship Shonan Maru.



Earlier in the day the fleet was contacted for the first time by the Ady Gil and Sea Shepherd's third vessel, the Bob Barker.



Mr Watson, aboard the society's bigger, but slower ship Steve Irwin, said he was still 500 nautical miles from the scene.



"This seriously escalates the whole situation," he said of the collision.



The Institute of Cetacean Research, which has previously fronted for the whalers, claimed the Ady Gil's crew were launching projectiles at a ship in the fleet, the Nisshin Maru, and attempted to entangle its propellers with rope.



"The research-base vessel Nisshin Maru, currently engaged in the Japanese whale research programme in the Antarctic ... was subject to attack today for about two hours by the New Zealand-registered watercraft Ady Gil," the institute said.



"In a manner similar to their 23 December attack on the Shonan Maru No. 2, at about (7am NZDT) the Ady Gil came to collision distance directly in front of the Nisshin Maru bow repeatedly deploying and towing a rope from its stern with the intent to entangle the Japanese vessel's rudder and propeller."



It said the crew of the New Zealand boat were also shining a green laser light and launching stinkbombs that smelled of rancid butter.



The Nisshin Maru started its water cannons "and proceeded to prevent the Ady Gil coming closer".



The institute claimed the activists' actions were "nothing but felonious behaviour" and potentially threatened the safety of Japanese sailors.



"In addition, their repeated deploying and abandonment of ropes designed to entangle the propeller of our navigating vessels ultimately ends up litter spoiling the Antarctic marine environment."

Japanese whalers have stepped up security this year, sending spy flights from Australian airports to track protest ships.

The Hobart flights were paid for by Wellington-based Omeka Communications, air industry sources said.

Omeka is a public relations firm retained by Japan's Institute of Cetacean Research. The Hobart flights carried Omeka's principal, Glenn Inwood, who is an institute spokesman, and another man, the sources said.

The operation started in December when the Steve Irwin left Fremantle to intercept the whaling fleet, which this year is targeting 935 minke whales and 50 fin whales.

Western Australian pilots said surveillance flights continued out of Albany for some days, costing a ''truckload'' of money. Two men aboard the flights told locals they were ''looking for people who were looking for whales''.

FACTS ON THE ADY GIL



Top speed: 40 knots (74 kmh)

Cruising speed: 20 knots (37 km/h)

Range: Halfway around the world - 20,000 km

Cost: Estimated $1 million

Length: 24 metres

Weight: 16 tonnes

Construction: Carbon fibre foam sandwich with kevlar armour.