Environmental activist group Sea Shepherd warned Japan against resuming "research" whaling in the Antarctic and called on the Australian government to intervene.
After a decade of harassment by Sea Shepherd and other groups, Japan was forced to abandon its 2014-15 Southern Ocean hunt after the International Court of Justice said the annual expedition was a commercial activity masquerading as research.
But on Saturday, Japanese media reported it would start again next year, despite a call by global regulators for more evidence that the expeditions have a scientific purpose.
"The pristine waters of the Southern Ocean are once again under threat from poachers," said Sea Shepherd chief executive Alex Cornelissen.
"We would like to remind the Japanese government that the whales of the Southern Ocean are protected by international law, by Australian law, and by Sea Shepherd.
"As such, any violation of the sanctity of the Southern Ocean Whale Sanctuary or the Australian Whale Sanctuary will be regarded as a criminal act," Agence France-Presse news agency quoted Cornelissen as saying.
During the suspension of Japan's whale hunt, Sea Shepherd has been targeting the catching of rare Antarctic and Patagonian toothfish in the Southern Ocean.
Its main ship, the Steve Irwin, is docked in Melbourne and the group did not say whether it would once again chase the Japanese whalers. The Yomiuri Shimbun and other media said the Japanese fleet could depart possibly by the end of December.
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Australia has led efforts to persuade Japan to halt whaling and Sea Shepherd called on Prime Minister Malcolm Turnbull to use diplomacy to ensure it does not resume.
Japanese public broadcaster NHK said Japan's Prime Minister Shinzo Abe was considering inviting Turnbull to Tokyo as early as December for a summit.
Australia has previously floated the idea of sending a vessel to monitor whaling in the Southern Ocean, but Foreign Minister Julie Bishop was non-committal on Sunday.
Despite international disapproval, Japan has hunted whales in the Southern Ocean under an exemption in the global whaling moratorium that allows for lethal research.
Japan makes no secret of the fact that meat from the mammals - killed ostensibly for research - is processed into food.