An Australian ship left Perth on Tuesday to monitor Japanese whaling in the Southern Ocean.

Oceanic Viking, which had 30 Customs officers on board, will spend 20 days gathering video evidence for a possible international court action on Japan's intention this season of catching 935 minke whales and 50 fin whales.

Initially, 50 humpbacks were to be taken as well but this catch is now in abeyance after Australia and New Zealand led an international protest.

The new Australian government led by Prime Minister Kevin Rudd has been accused of talking tough on the whale slaughter but doing little about it.

Foreign Minister Stephen Smith said before Christmas that Oceanic Viking would leave "within days" to track the four-vessel Japanese whaling fleet as it made its way to the Southern Ocean. He also promised aerial surveillance, but this too has yet to materialize.

A spokesman for Japan's whaling mission said this week the fleet was already mid-way through its Southern Hemisphere summer hunt.

"They wanted to come out heavy, to be seen to be doing something, and they have not followed through," Greens member of parliament Rachel Siewert said of the government's belated mission to the whaling grounds.

Opposition Liberal Party environment spokesman Greg Hunt said Rudd had shown he was not serious about the issue but engaging in "domestic posturing."

International environmental lobby group Greenpeace called on the government to send up a plane to pinpoint the location of the fleet, so its ship, the Esperanza, could begin harassing the Japanese. The Esperanza has been in Antarctic waters since January 2 searching for the fleet.

The Sea Shepherd Conservation Society has also sent a boat, the Steve Irwin, to find the Japanese.

"We are pleased that the Oceanic Viking has finally left and look forward to seeing her in the Southern Ocean," Greenpeace whales campaigner Rob Nicoll said in a statement.

"Of course, if the Australian government already has the location of the fleet, then we would like those coordinates as we are much closer than the Oceanic Viking. However, we do not expect them to give them to us."