The fleet aims to catch more than 1,000 whales before returning to port early next year, Greenpeace has said.
"It's clearly commercial whaling in disguise and the aim for the Japanese government is to restart commercial whaling," Karli Thomas, leader of the Esperanza, Greenpeace's campaign ship.
"The whaling fleet must be recalled now. If it is not, we will take direct, non-violent action to stop the hunt," she said.
'Viciously cruel'
The Esperanza is aiming to intercept the fleet in Japanese waters in an attempt to stop the expedition.
About 100 humpback and fin whales, which environmentalists say are both vulnerable species, are among the 1,000 whales that have been ordered to be hunted.
The remainder will consist of minke whales.
The Sea Shepherd Conservation Society said Japan was "viciously cruel" for hunting humpbacks, and has pledged to stop the mission.
"Sea Shepherd will be hunting the whalers with the firm objective of intervention against their illegal activities," a Sea Shepherd statement said.
The decision by Japan to hunt humpbacks is expected to draw criticism from Australia, which has already said that relations between the two countries could worsen if the hunt goes ahead.
'Scientific research'
About 1.5 million whale watchers observe the northern migration of humpbacks along Australia's coast to breed each year, pumping an estimated $225m into the Australian economy.
"It's important that Japan understands that the inclusion of humpbacks will have an impact on perceptions of Japan in Australia," Malcolm Turnbull, Australia's environment minister, said earlier this year.
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Nisshin Maru is leading the Japanese whaling
fleet after repairs from a fire [AFP] |
Japan says its whaling programme is carried out for scientific research and aids the understanding of whale stocks and species.
Fisheries officials have in the past protested against the activities of environmental organisations.
Japan, which kills more than 1,000 whales a year in the Antarctic and the Pacific Ocean, abandoned its last Antarctic whale-hunting season in March after fire broke out on the Nisshin Maru, killing one crew member.
Japan observed an international moratorium and halted commercial whaling in 1986, but then resumed the following year.
Norway and Iceland defy the moratorium outright.
Whale meat is sold for consumption under rules set by the International Whaling Commission, although demand among Japanese consumers has been falling.