Australia and New Zealand on Tuesday hailed a landmark court decision that Japan must halt an annual Antarctic whale hunt, despite fears it may try to sidestep the order.
The United Nations' Hague-based International Court of Justice on Monday ruled that Japan's whaling programme was a commercial activity disguised as science and said it must revoke existing whaling licences.
A "deeply disappointed" Tokyo said it would honour the ruling but did not exclude the possibility of future whaling programmes, with New Zealand expressing concerns Japan may try to circumvent the order.
"The ICJ decision sinks a giant harpoon into the legality of Japan's whaling programme," New Zealand Foreign Minister Murray McCully said.
"It still does leave Japan with a decision to make after they've digested this which is to look at whether they try to devise a new programme that is scientifically based that they could embark upon whaling in the Southern Ocean again.
"Our task is to make sure that we carry out a diplomatic conversation that dissuades them from embarking on that course."
A Japanese minister on Tuesday defended whaling -- seen by some as an important cultural practice -- and pointedly stopped short of detailing what next steps Japan could take.
"Whale meat is an important source of food, and the government's position to use it based on scientific facts has not changed," Agriculture, Forestry and Fisheries Minister Yoshimasa Hayashi told a press conference.
"We will scrutinise the verdict and study (measures to be taken) swiftly," he said according to the Jiji news agency. Japan also has a coastal whaling program which is not covered by the ban.
Australia, backed by New Zealand, hauled Japan before the ICJ in 2010 a bid to end the annual Southern Ocean hunt with Tokyo accused of exploiting a legal loophole in the 1986 ban on commercial whaling that allowed the practice to collect scientific data.
International law expert Steven Freeland, from the University of Western Sydney, said Japan could simply redesign its whaling programme to skirt the ruling, pointing out that the ICJ confirmed scientific research can include killing whales, just not as many.
"The problem for Japan was its failure to take proper account of non-lethal methods of research or to justify the actual catch numbers it had declared," he said.
"Japan may instead take a very close look at why its implementation of (its research programme) fell foul of its legal obligations and perhaps seek to design and ultimately implement a new whaling programme that takes into account all of those elements."
Japan had argued that its JARPA II research programme was aimed at studying the viability of whale hunting, but the ICJ found it had failed to examine ways of doing the research without killing whales, or at least while killing fewer of them.
Former Australian attorney-general Mark Dreyfus, who was involved in arguing the case at The Hague, agreed that "on paper" the judgment left the door open for more slaughter.
"I'm calling on the government to work with Japan on non-lethal research, so that there won't be killings of whales in the Southern Ocean ever again," he said.
Australia's current Attorney General George Brandis was subdued in his response, possibly mindful of the fact that Prime Minister Tony Abbott will next week make his first trip to Japan since winning office.
However, former environment minister Peter Garrett said he felt vindicated.
"I'm absolutely over the moon, for all those people who wanted to see the charade of scientific whaling cease once and for all," said Garrett, who helped launch the legal action -- the first time a country had used an international court to try to stop whaling.
The decision came ahead of Abbott heading to Tokyo in a bid to finalise a free trade agreement with Australia's second-largest trading partner.
When asked if the trade deal could be derailed by the whaling fallout, Brandis said "I'm sure it wouldn't".
"The fact that Australia and Japan were able to be in dispute on this narrow issue... and to agree to differ, but to nevertheless maintain an excellent relationship... I think demonstrates very clearly what a strong relationship and what an enduring relationship this is," he said.