SYDNEY (AFP) – An anti-whaling activist has boarded a Japanese security ship to make a citizen's arrest of its captain over last month's sinking of a high-tech boat, an animal rights group said on Monday.

New Zealand's Pete Bethune, captain of the futuristic Ady Gil powerboat which was smashed in two on January 6, jumped aboard the Japanese ship from a jetski under the cover of darkness, the Sea Shepherd group said.

"This was an impossible mission," spokesman Paul Watson said in a statement.

"Captain Bethune boarded a Japanese whaling fleet security ship at high speed in total darkness, breached the spikes and anti-boarding nets and is presently onboard and the Japanese crew are still not aware that he is there.

"He is there to demand justice for the sinking of his ship."

Sea Shepherd said Bethune would try to arrest the Shonan Maru 2's captain for "the destruction of the Ady Gil and attempted murder of the six Ady Gil crewmembers".

He would also attempt to hand over a three million US dollar bill for the destruction of the state-of-the-art trimaran, which set the round-the-world powerboat record under its former name, Earthrace.

"Sea Shepherd anticipates that the Japanese will hold Captain Bethune as prisoner onboard the Shonan Maru 2," the group's statement added.

The activists claim to have saved the lives of hundreds of whales by harassing the annual hunt, which kills the animals using a loophole in an international moratorium which allows "lethal research".

Australia and New Zealand have called for restraint from both sides in their Antarctic clashes, with the activists deploying laser-like devices and stink bombs and the Japanese fleet operating military-style acoustic weapons.