JAPAN has pleaded with the Australian government to prevent Sea Shepherd's anti-whaling vessels from leaving port.

Department of Foreign Affairs and Trade secretary Dennis Richardson says Japan sometimes asks Australia to do things it cannot do.

"They have for instance asked us at times to prevent the Sea Shepherd leaving Australian ports," Mr Richardson told a Senate estimates hearing.

"And we can't legally do that."

Australia would not intervene to stop peaceful protest, he said.

"We have stated that to the Japanese on numerous occasions," Mr Richardson said.

The Japanese government last week confirmed it was cutting short this season's whale hunt, citing harassment from the Sea Shepherd Conservation Society.

Activists from the US-based Sea Shepherd have pursued the Japanese fleet for weeks to keep its harpoon ships from killing whales.

Mr Richardson said Japan's announcement was "encouraging".

"But I think it would be premature to jump to a view that it reflects a change in policy," he said.

Originally published as Japan seeks help on Sea Shepherd