Two ships have left Australia bound for the freezing Southern Ocean to confront the Japanese whaling fleet in an annual high-seas battle, the environmental activist group Sea Shepherd has said.

The organisation’s flagship, Steve Irwin, departed for Antarctic waters on Monday along with a fast new patrol vessel, Ocean Warrior, built with financial support from the Dutch, British and Swedish lotteries.

It has a powerful water cannon and is capable of outrunning the whalers, which an official at Japan’s Fisheries Agency said would be protected by a fleet of patrol boats.

“Sea Shepherd has engaged in repeated acts of sabotage over the years. Those actions threaten the lives of Japanese crew members and we cannot tolerate it,” said the official, who declined to give his name.

Japan has previously sought court action to halt the anti-whaling campaigns, saying the activists ram their ships, snare propellers with ropes, and harass crew with paint and stink bombs.

Sea Shepherd is embarking on its 11th campaign to disrupt the hunt. The Japanese fleet set sail on 18 November in defiance of a worldwide moratorium on commercial whaling and an international court ruling.

“It’s time that Japan respected the International Court of Justice ... and the global moratorium on commercial whaling and ended their so-called scientific lethal hunting of whales off the Antarctic coast,” said Sea Shepherd Australia’s chief Jeff Hansen.

Japan is a signatory to the International Whaling Commission’s moratorium in force since 1986. But it uses a loophole that allows whales to be killed for the purposes of scientific research.

Tokyo claims it is trying to prove the whale population is large enough to sustain a return to commercial hunting for a traditional source of food. But the meat from what it calls scientific research still ends up on dinner tables and is served up in school lunches.

In 2014 the United Nations’s international court of justice ordered Tokyo to end the Antarctic hunt, saying it found permits issued by Japan were “not for purposes of scientific research”.

After the ruling Japan cancelled its 2014-15 hunt, only to resume it the following year under a new programme with a two-thirds cut in the target catch number, saying the fresh plan is genuinely scientific.