Defiant ... Paul Watson and the ship Steve Irwin. A US court has issued an injunction.

Defiant ... Paul Watson and the ship Steve Irwin. A US court has issued an injunction. Photo: Supplied

ACTIVIST Paul Watson says a US court injunction ordering his Sea Shepherd group to keep away from the Japanese whaling fleet will not deter its Southern Ocean campaign.

A US appeals court this week ordered Sea Shepherd and Mr Watson not to physically attack or endanger the whaling ships, and to stay at least 450 metres away at all times.

The order was sought by the Institute of Cetacean Research (ICR), which undertakes the Japanese government's whaling program through a loophole in International Whaling Commission rules that allows whales to be killed for research.

The ICR has welcomed the injunction, saying Sea Shepherd has engaged in ''violent and unlawful attacks''.

Mr Watson, the head of the anti-whaling group and currently aboard its vessel Steve Irwin, said Sea Shepherd was confused by the decision but steadfast in its resolve.

''We are 100 per cent committed to saving whales down here, that's why we're calling it Operation Zero Tolerance,'' he said on Wednesday.

''This campaign requires courage, passion and imagination - now it's time to throw some imagination into it."

He said the Japanese had engaged in aggressive acts against his group without being held to account, and lawyers for Sea Shepherd would look at appeal options.

An Australian National University international law professor, Donald Rothwell, said the order would be almost impossible to enforce, but could create problems for Sea Shepherd in the future.

''Because Sea Shepherd is a registered company in the United States and has its headquarters in the state of Washington, Sea Shepherd would be subject to consequences under US law if it failed to abide by the injunction,'' he said on Wednesday. ''Paul Watson does hold a US passport and could be held in contempt of court and arrested in the US.''

Acting Greens Leader Adam Bandt condemned any violent protest, but said Sea Shepherd was the only group monitoring whether international whaling laws were being complied with.

''We should be clear the right to peacefully protest is not only paramount, but the Sea Shepherd is the only one out there attempting to uphold the law.''

AAP

Correction: The headline on the original version of this story was "Sea Shepherd activist vows to defy court over whaling fleet".