The government has expressed "extreme disappointment" over Japan's continued efforts to overturn a long-standing moratorium on commercial whaling.

Japan has passed legislation that commits to subsidising its whaling fleet, which Foreign Minister Gerry Brownlee said was a step backwards for the conservation of whales.

"New Zealand will continue to reinforce our support for the moratorium on commercial whaling, which has been in place for over 30 years," Brownlee said.

"The recent departure of Japan's whaling fleet to the western North Pacific once again goes against the recommendations of the International Whaling Commission, its scientific committee and its expert panels.

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"Japan is persistently failing to justify the need to kill whales in the name of research, in both the North Pacific and the Southern Ocean."

In January New Zealand expressed its "significant disappointment" to Japan over the country's whaling in the Southern Ocean.

The International Court of Justice ruled against Japan's whaling operations in the Southern Ocean in 2014, after a case brought by Australia.