Three activists who sparked a diplomatic incident by boarding a Japanese whaling ship arrived back in Australia on Monday as Prime Minister Julia Gillard blasted their "irresponsible" behaviour.

The men from the Forest Rescue Australia environmental group clambered on board the Shonan Maru No.2, escorting Japanese whalers on an Antarctic hunt, off Australia's west coast on January 7.

There were fears they would be taken to Japan and tried for trespassing but Tokyo agreed to release them and they were transferred to an Australian customs ship on Friday, arriving in the west coast port of Albany on Monday.

The West Australian newspaper said they were escorted from the vessel by Australian Federal Police officers to cries of support from a small band of supporters.

But Gillard said they should not be treated as heroes despite Canberra's firm opposition to Japanese whaling in the Southern Ocean, with the operation to retrieve them costing taxpayers hundreds of thousands of dollars.

"The truth is, it has been a costly venture to go and retrieve these three men," Gillard told reporters in Hobart, adding that it was unlikely the activists would contribute to the cost.

"I do not understand them to be people who probably have that kind of money at their disposal, so unfortunately, it is the government, the Australian taxpayer, who to date, has had to bear these costs, which is why, among other reasons, I think their conduct was irresponsible."

Gillard said she was as anti-whaling "as anybody else in this country" but believed the most effective course was to take the matter to the International Court of Justice, which Australia was doing.

Attorney-General Nicola Roxon reminded Japan that its whaling vessels were not welcome in Australian waters, amid reports last week that one had strayed into Australian territory around sub-Antarctic Macquarie Island.

"The Australian government continues to condemn Japan's decision to continue its so-called 'scientific' whaling in the Southern Ocean this summer," she said.

"And it reiterates its request for whaling vessels to stay out of Australia's territorial sea and exclusive economic zone."