Tokyo, Japan (CNN) -- Japanese authorities arrested an activist from New Zealand on Friday for illegal trespassing after he boarded a whaling ship last month in waters off the Antarctic.

Peter James Bethune, 44, is accused of jumping from a Jet Ski onto the Shonan Maru 2, the security ship of a Japanese whaling fleet, after the Ady Gil boat, of which Bethune was captain, sank.

Bethune belongs to the activist group Sea Shepherd Conservation Society. The group said the New Zealander was attempting to make a citizen's arrest of the Shonan Maru 2 skipper for the collision that sank the Ady Gil, a futuristic vessel used to intercept and block harpoon ships, in January.

Hirotaka Akamatsu, Japan's minister of agriculture, forestry and fisheries, said Japan would take a firm stance against Bethune. He said Sea Shepherd's acts were not acceptable.

"Their violent acts are escalating," Akamatsu said.

Bethune was taken into custody a month ago aboard the Shonan Maru 2, which arrived from Antarctic waters -- where Japan conducts its annual whale hunt -- back to Japan on Friday. He was formally arrested then.

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Bethune has legal representation in Japan, said Sea Shepherd spokeswoman Traci Walter.

A spokesperson for the New Zealand Ministry of Foreign Affairs and Trade said the government was providing consular assistance to Bethune on his arrival in Japan.

The Bethune case highlights an ongoing feud between Japanese whaling fleets and conservation agencies, especially the hardline Sea Shepherd.

Activists have used butyric acid -- found in rancid butter and vomit -- and fired paint guns at Japanese whaling ships, which in turn have repelled protesters with water cannons.

Japan's Institute of Cetacean Research, a branch of the fisheries ministry that deals with whaling, accuses Sea Shepherd of jeopardizing the safety of fleets that are conducting research legally.

Sea Shepherd uses its boats to interfere with whaling and fishing boats, and its efforts have included ramming a Portuguese whaler in 1979.

Two years ago, Sea Shepherd activists boarded a Japanese ship and handcuffed themselves to the vessel with plastic ties.

Japanese authorities had called the Shonan Maru 2 incident the latest "publicity stunt" by Sea Shepherd activists.

Sea Shepherd has accused the Shonan Maru 2 of destroying the Ady Gil during the skirmish in the Southern Ocean -- a term used to describe parts of the Indian, Pacific and Atlantic oceans that surround Antarctica.

However, Japanese authorities deny their ship intentionally hit the high-tech catamaran and have said the activists were harassing their whaling fleet.

In the early 1980s, the International Whaling Commission determined that there should be a moratorium on commercial whale hunting. But whaling is allowed under international law when done for scientific reasons, which Japan cites as the legal basis for its hunts.

The country's annual hunt kills up to 1,000 whales a year.