Federal Environment Minister Peter Garrett has denied rushing through a $4 million anti-whaling package as a Japanese whaling fleet sets sail on its annual Antarctic hunt.

Greenpeace monitors said the fleet's factory ship, the Nisshin Maru, left the port of Innoshima in western Japan on Monday.

The Federal Opposition said Mr Garrett reacted to that news by "frantically" announcing the research project because "he wanted to give the impression that he was doing something proactive about whaling".

"All he did was to waste millions of taxpayers' dollars for research into what we already know - that the Japanese whale research program is a sham. It's research into sham research," shadow environment minister Greg Hunt said in a statement on Tuesday.

Mr Garrett fired back, warning the Federal Opposition to be "very cautious" in its criticism of the Government's measures.

"Over a period of 10½ years, they created a lot of sound and fury, didn't produce any substantial reform and saw a doubling of the targets for the whaling operations by Japanese whalers in the Southern Ocean," Mr Garrett said on Tuesday.

"The Opposition has very little credibility on this issue."

Mr Garrett said he was also disappointed at reports the Japanese fleet had set sail, but he reiterated that the Government would make a decision "reasonably soon" on whale monitoring.

He added he was confident the $4 million research program would achieve results, despite its lofty aim of forcing Japan eventually to stop hunting whales under the guise of "lethal research".

"I'm not under any delusions as to how difficult and challenging this task is, but ... we need to continue through with a proactive, deliberative and policy-strong approach to both reform of the IWC [International Whaling Commission] and to non-lethal whale research."

AAP