A six-vessel whaling fleet took off from the western port of Shimonoseki in Japan for its five-month voyage on Sunday heading to the Antarctic Ocean for a hunt that will include humpback whales for the first time.
The Sea Shepherd Conservation Society says it will physically stop the hunt by ramming whaling vessels if necessary.
"Basically we're going down there to stop them," Sea Shepherd's Jonny Vasic told Channel 9 of Australia this morning.
"We're not going down there to protest; we're going down to directly intervene and put an end to this criminal behaviour.
"We've been known to ram a vessel that's engaged in illegal activity as a last ditch effort to get them to stop.
"We have a reputation of direct action and we mean business.
"We, too, are a non-violent group, but we have a problem with economic destruction when it's engaged in illegal activity."
Karli Thomas from the environmental group Greenpeace, said they too were determined to stop the hunt once the fleet reached the Southern Ocean, but would not go to the same lengths as Sea Shepherd.
"We will do what we can in the bounds of non-violent direct action to stop them hunting whales," Ms Thomas said.
"So that means putting ourselves between the harpoon and the whales, preventing them from taking a shot, and being able to save individual whales from the harpoon.
"We certainly have never rammed a vessel, and we don't intend to."
During the last expedition, Sea Shepherd activists threw bottles of chemicals at the whalers in hopes of disrupting them.
The group was also involved in a ramming incident with the Japanese whalers.
Humpback whales have been protected under a 1966 worldwide moratorium after years of overhunting.
Japan, which argues that eating whale meat is part of its culture, planned to kill 950 whales on the five-month mission using a loophole in a global moratorium that allows lethal research on the giant mammals.
Australia, Britain and New Zealand have condemned the decision to go ahead with the catch.