The federal government, acting under heavy diplomatic pressure from Japan, explored taking legal action against the anti-whaling group Sea Shepherd.

Tokyo angrily strengthened demands for Australia's co-operation against further ''sabotage'' of its whaling fleet in the Southern Ocean after the Ady Gil collision, declassified documents show.

In 2009 and last year the government considered grounds for legal action. It found it could not ban Sea Shepherd ships from Australian ports or detain them.

However, both the Department of Foreign Affairs and Trade and the Japanese embassy, using an international convention against terrorism at sea, required the Australian Federal Police to undertake a criminal investigation of Sea Shepherd's activities. This investigation is continuing.

Revelations of the Foreign Affairs liaison with Japan follow a disclosure by WikiLeaks that a Foreign Affairs official was confident the whalers would ''come away clean'' from the collision in which the security ship Shonan Maru No. 2 sliced the bow off Sea Shepherd's Ady Gil.

The only official investigation to attribute responsibility for the shipwreck, by Maritime New Zealand, recorded black marks against the skippers of the Ady Gil and the Shonan Maru No. 2.

Censored Foreign Affairs documents were released to the Greens leader, Bob Brown, on appeal in a freedom-of-information request, after he was denied access to papers relating to federal police searches of Sea Shepherd ships.

''This shows a double standard that puts Japan above Sea Shepherd,'' Senator Brown said. ''It's a disgraceful commentary that cuts across common legal prudence.''

The Foreign Affairs documents show a flurry of diplomatic activity after the collision last January. Japan demanded action against Sea Shepherd ships when they returned to Australia to refuel during their campaign and again at the end of the whaling season.

Talks reached a climax during a protest to the Foreign Affairs secretary, Dennis Richardson, in Canberra by the visiting Japanese state secretary for foreign affairs, Tetsuro Fukuyama.

A senior legal adviser to the Australian department, Penny Richards, said in a confidential ministerial submission that Australia was obliged to act under the Convention for the Suppression of Unlawful Acts against the Safety of Maritime Navigation, because the Ady Gil's crew had returned to the country.

''We are not making a judgment about whether the Ady Gil was responsible for the collision with the Shonan Maru No. 2 but the fact of the collision makes both crews 'alleged offenders' for the purposes of the SUA convention,'' she said.

She said the Foreign Affairs and the Attorney-General's departments told the AFP it should mount a criminal inquiry, and Japan followed up separately.