The aerial footage released by the Sea Shepherd Conservation Society appeared to show the bloodied remains of three minke whales on board the Japanese factory ship Nisshin Maru as it sailed in the Southern Ocean. The group said it had information a fourth whale had also been killed.
The images have raised pressure on Environment Minister Greg Hunt, who has backed away from a promise made during Australia's election campaign last year to send a ship to monitor Japanese whaling in the area.
His plan to instead send an aircraft staffed by customs officials to randomly swoop over the area has been dismissed by activists as ineffectual.
Bob Brown, chairman of Sea Shepherd Australia, called on the conservative Liberal Party-led government to honour its pledge to step up monitoring of the Japanese fleet.
He said: "We've got a federal court ruling that this whaling is illegal and an injunction to stop it."
Mr Hunt's office dismissed the criticism as political manoeuvring and said it would still monitor whaling.