Conservation group Sea Shepherd Australia said Monday it was preparing for increased aggression from Japanese whalers in its annual campaign to stop the slaughter of the giant animals off Antarctica.

Three Sea Shepherd boats are due to leave from Melbourne and Hobart within days to harass and deter the Japanese harpoon ships which hunt minke, fin and humpback whales in the southern hemisphere summer.

"We're definitely ready to leave," said Siddharth Chakravarty, captain of the Steve Irwin, the boat formerly skippered by Sea Shepherd Conservation Society founder Paul Watson who is prohibited from taking part this year due to a US court injunction.

The Steve Irwin had been due to leave Melbourne early Monday but was held up by a delayed fuel delivery and is now aiming to depart later in the day.

Fellow Sea Shepherd vessel the Sam Simon is set to depart the city Tuesday while the Bob Barker will leave from the southern island state of Tasmania on Wednesday.

Sea Shepherd campaigners have chased the Japanese fleet for a decade to stop whales being slaughtered, and Chakravarty said the violence was escalating.

"What can definitely be expected is that there will be a lot of aggression from the Japanese whaling side," he told AFP.

"Every year they have been escalating their violence against Sea Shepherd.

"We are obviously there to stop them from whaling and they are frustrated and angry about this. But we've got one commitment which is to the whales and... making sure that as few whales as possible are taken."

High-seas clashes between the groups are common and last year Sea Shepherd claimed that a Japanese boat had rammed its vessels on multiple occasions -- destroying masts and a radar on the Bob Barker and leaving it without power.

The Japanese claimed their boats had been rammed by the campaigners in what was the worst confrontation in the Southern Ocean since the January 2010 collision in which Sea Shepherd's Ady Gil sank.

Chakravarty said the Japanese vessels were expected to reach Antarctica sometime between Christmas and New Year's Day.

"We are timing our departure to be there before them so that we can start looking for them and make sure that we get through December without a single whale being killed," he said.

In recent years the Japanese have left Antarctica without filling their quota but Chakravarty said even under this scenario the Sea Shepherd boats would not be back in port until March.

Japan says it conducts vital scientific research using a loophole in an international ban on whaling, but makes no secret of the fact that the mammals ultimately end up as food.

Australia wants the practice to stop and has taken the matter to the UN's top court the International Court of Justice. A decision is expected in early 2014.