New Zealand won't join in Australian court action over Japanese whaling but will have input into the case.

The Government has decided not to file as a party to Australia's legal action in the International Court of Justice, the United Nations' highest court, against Japanese 'scientific whaling' in the Southern Ocean.

But it will have input into the case, Foreign Minister Murray McCully and his Australian counterpart Kevin Rudd said in a joint statement.

In June, Australia launched a complaint against Japanese whaling at the court in The Hague.

Commercial whaling was banned worldwide in 1986, but Japan set up the non-profit Institute of Cetacean Research a year later and has since culled hundreds of the ocean mammals annually for scientific research.

Japan argues its scientific activity is permitted under the International Convention for the Regulation of Whaling, but Australia says it is a commercial activity.

Mr McCully said that the Cabinet had this week agreed to his recommendation to intervene in the case but not to file as a party.

"Australia has indicated that they would prefer New Zealand not to file as a party. Because New Zealand has a judge on the ICJ, Sir Kenneth Keith, the joining of the two actions would result in Australia losing its entitlement to appoint a judge for the case. New Zealand's decision to intervene will allow the case to proceed without delay," Mr McCully said.

"With this decision made, we have begun to focus on new diplomatic and communications strategies to try to persuade Japan to end whaling in the Southern Ocean. With this in mind, I have spoken to Japan's Foreign Minister Seiji Maehara to explore the room for further diplomatic initiatives."

Mr Rudd said New Zealand's decision was what Australia wanted.

"New Zealand has once again confirmed that it is a strong partner of Australia in the bid to end scientific whaling and improve whale conservation worldwide," Mr Rudd said.

"By intervening in the case, New Zealand will be able to make both written and oral submissions to the court that Japanese whaling in the Southern Ocean is contrary to its obligations under applicable international conventions to which Australia and New Zealand are also Parties.

"We have kept in close consultation with the Government of New Zealand about how best to progress our shared anti-whaling objectives. We are very pleased with the valuable support New Zealand will lend to this vital case."