Prime Minister John Key says Japan's decision to resume whaling in the Southern Ocean is "appalling" and a show of "bad faith" and he would raise it with Japan's Prime Minister Shinzo Abe if they met in Paris.

Mr Key is at the CHOGM in Malta but heads to Paris early this morning for the COP21 climate talks.

Japan has advised the International Whaling Commission it will resume whaling in the Southern Ocean but has reduced its intended take.

That is despite an International Court of Justice ruling in 2014 that Japan's previous whaling programme was unlawful because it was not for genuine scientific research.

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Mr Key said the New Zealand Government had expressed its displeasure to Japan. He would raise it personally with Mr Abe if they met in Paris.

"These actions are in bad faith and we are appalled by the decision. There has been a long process and legally correct process we followed through the ICJ and Japan are just going completely around that decision. They haven't made any attempt to justify the resumption as scientific rather than [commercial] whaling dressed up as scientific."

Mr Key said he had discussed it with Australia's Prime Minister Malcolm Turnbull in Malta.

Australia has also condemned the decision and took the case to the ICJ, which New Zealand supported.

He said the Government was weighing up the options it had, including legal avenues.
The Green Party has called for the Government to send surveillance air craft or vessels to monitor the whaling and Mr Key said that was an option but it was problematic because it was in international waters.

Labour's foreign affairs spokesman David Shearer said he was disappointed by the decision. "'Scientific whaling' is just a cover for whaling generally. It will be disappointing for many countries."

He said New Zealand should continue to condemn the programme both privately and publicly.