Any deal with Japan to allow whaling would have to be very attractive to get New Zealand support, Foreign Minister Murray McCully says.

Australian Prime Minister Kevin Rudd has repeated his threat to take Japan to the International Court of Justice (ICJ) if a diplomatic solution is not reached.

Talks in the United States over the weekend of International Whaling Commission nations failed to agree on a compromise proposal.

Australia wants whaling to be phased out in five years but New Zealand is among those seeking a compromise.

The proposal under negotiation would allow Japan, Norway and Iceland to openly hunt whales despite a 1986 moratorium on commercial whaling, but aim to reduce the total catch over the next 10 years. Japan currently uses a loophole to kill whales saying it is for scientific research.

Anti-whaling group Sea Shepherd said New Zealand had given into extreme pressure from Japan.

"This Government should figure out who it represents, the people of New Zealand or the people of Japan," Sea Shepherd captain Paul Watson said.

Mr McCully said the Government wanted whaling in the Southern Ocean eliminated and he thought the best way of getting substantial reductions was through the diplomatic process.

"There is no mandate for the New Zealanders who are participating in the discussion to do any deal whatsoever. The only mandate they have is to see if they can find a diplomatic solution that the New Zealand Government and then the New Zealand people can consider," he told Radio New Zealand this morning.

"If there is a suggestion of a diplomatic solution we will obviously tell New Zealanders what's on the table and seek their views. If it's something that's worth thinking about it will involve some difficult decisions because it would involve obviously accepting a reduced amount of whaling for a period of time rather than an immediate elimination that's always been the nature of that discussion."

The reduction would have to be significant, he said.

New Zealand International Whaling Commission (IWC) representative Sir Geoffrey Palmer told RNZ New Zealand and Australia had the same aims and it would be "enormous progress" to remove the scientific whaling loophole.

The compromise would bring scientific whaling under the control of the IWC, requiring Japan to submit DNA samples and other data to the 88-nation body.

Norway and Iceland defy the moratorium on commercial whaling altogether by lodging objections to the international decision, a practice that would be banned under the compromise.

Sir Geoffrey said all diplomatic processes needed to be explored before going to the ICJ.

"It's a very uncertain prospect that the case has, and if it was lost I think the chances of making progress in the future would be very limited indeed. It's quite possible that the International Whaling Commission could break up altogether.

"We are particularly interested in New Zealand in saving the International Whaling Commission because we thinking that's the best way of saving whales."

Australian Green Party leader Bob Brown said both Australia and New Zealand needed to act immediately and take Japan to the ICJ.

Mr Brown said it looked like both countries would cave in and agree to some form of commercial whaling against the wishes of the people.

"The whaling ought to be injuncted," he told RNZ.