New Zealand led an international protest Monday against Japan's plans to resume killing whales in the Southern Ocean, while Australia said it was considering further legal action.

Prime Minister John Key said New Zealand's ambassador to Tokyo delivered a "strong" formal message from 33 countries, including the United States and Australia, to Japan.

"We consider that there is no scientific basis for the slaughter of whales and strongly urge the government of Japan not to allow it to go ahead," Key said in a statement.

A Japanese whaling fleet set sail for the Antarctic last week, despite environmentalists slamming the move as a "crime against nature".

Tokyo said last month it planned to kill 333 minke whales for scientific research this season, despite a worldwide moratorium and opposition led by Australia and New Zealand.

The fleet's departure marked the end of a year-long suspension prompted by a United Nations' International Court of Justice (ICJ) ruling in 2014 that the annual hunt was a commercial venture masquerading as research.

Australia, which hauled Japan before the ICJ in 2010 in a bid to end the annual hunt, would continue to raise concerns, Foreign Minister Julie Bishop said.

"We are working with other like-minded nations to build international consensus against Japanese whaling," Bishop said in a statement.

"We are also exploring options for further legal action."

Tokyo claims it is trying to prove the whale population is large enough to sustain a return to commercial hunting, and says it has to kill the mammals to carry out its research properly.

However, it makes no secret of the fact that the animals' meat ends up on the dinner table or served up in school lunches.

New Zealand, which said Mexico, South Africa and EU member countries also joined the formal protest, said it would press for an end to "this outdated practice".

Australia made clear lethal research was unnecessary.

"The science is clear: all information necessary for management and conservation of whales can be obtained through non-lethal methods," Bishop said.

Environmentalists from Sea Shepherd Australia have vowed to pursue the Japanese fleet and aim to intervene in any slaughter of the animals, as it has done for the past decade.

Their vessel the Steve Irwin was due to leave Melbourne on Monday evening for Western Australia, before it leaves for the Southern Ocean on a mission to halt illegal fishing.

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