Japan is not planning to walk out of the International Whaling Commission (IWC) simply because the UN's top court has ruled its Southern Ocean hunting program is illegal.

In a surprise judgment, the International Court of Justice on Monday backed a landmark case put by Australia and demanded Japan cease its whaling program "with immediate effect" as it didn't comply with the country's obligations under the 1946 whaling convention which established the IWC.

During a three-week hearing in mid-2013, a lawyer acting for Japan argued Canberra's opposition to whaling was "an emotional anti-whaling moral crusade that in the name of zero-tolerance tolerates (activist group) Sea Shepherd's violent extremism, the politicisation of science, the collapse of the IWC and now, before this court, baseless accusations of bad faith against Japan".

Professor Payam Akhavan warned that countries could quit the 88-member whaling commission as a result - leading to a lack of regulation.

But Japanese delegation spokesman Noriyuki Shikata on Monday insisted Tokyo wasn't about to quit the IWC.

"We have been very actively engaged in discussion at the IWC and we wish to continue to engage ourselves in the IWC," he told AAP in The Hague.

"That's the only forum focused on whaling issues and it is the appropriate forum to discuss the conservation and management of whaling stocks.

"I don't think we have talked anything about leaving the IWC. We have not said anything along that line."

Japan's chief negotiator at the ICJ, Koji Tsuruoka, stressed Tokyo had been an active member of the commission for over 60 years.

He said although it had been riddled by internal divisions, Japan had maintained a constructive role within the IWC "and we are hoping that we will be able to continue to work in this spirit with the IWC".

Asked whether Japan would therefore rule out abandoning international agreements, Mr Tsuruoka said: "Please don't put words into my mouth."

He said officials would read Monday's court decision carefully and only then "consider what appropriate course of action we should take".

Source: ABC