Anti-whaling group the Sea Shepherd Conservation Society says it will halve the Japanese slaughter of whales in the Southern Ocean this summer.

Sea Shepherd founder and president Paul Watson says the anti-whaling fleet will leave Hobart tomorrow - beating the Japanese whalers there for the first time.

"I can guarantee we're going to cut their quota in half, but we're aiming for 100 per cent this year.

"We're prepared for their aggression, we're stronger and better prepared and better equipped than ever," he said.

The two Sea Shepherd vessels the Bob Barker and Steve Irwin will be joined in Hobart by the group's newest interceptor Gojira today and set sail for the Southern Ocean tomorrow.