Australia will take diplomatic action to encourage Japan to drop its "scientific whaling" program, amid reports the Japanese fleet has begun its journey south.

Environment Minister Tony Burke has restated the government's strong opposition to Japan's whaling expeditions and dismissed Japanese claims that the program is for scientific research.

"There is nothing scientific about going out and chasing whales, aiming a harpoon at them, so that you can pull them in and chop them up for food," Mr Burke told reporters in Sydney on Saturday.

Australia would lobby Japan to respect a moratorium on whaling in the Southern Ocean, taking "all diplomatic action that a government can take", he said.

But Mr Burke said the government would not respond to Coalition calls for a customs vessel to be sent to the Southern Ocean to monitor the annual whale hunt.