Sea Shepherd claims to have found Japanese whalers trying to cover up a slaughtered whale on the deck of a ship in protected Australian waters.
On Sunday, the Nisshin Maru was spotted with a minke whale on its deck by a Sea Shepherd helicopter from the group's flagship MY Steve Irwin.
In 2014 the International Court of Justice (ICJ) ruled the Japanese whaling program illegal.
The Australian Federal Court also found the Japanese whaling industry in contempt for killing protected whales in the Australian Whale Sanctuary.
The photos, published by Sea Shepherd, are the first to be documented since the ICJ ruling.
The environmental group also claims the ship's crew tried to cover up the whale's body with a tarpaulin as soon as they realised they were being watched.
"The whale-killers from the Nisshin Maru were caught red-handed slaughtering whales in the Australian Whale sanctuary," says Captain Adam Meyerson of the Ocean Warrior.
"The Steve Irwin has shut down their illegal operations and caught them trying to hide the evidence."
He says the fleet's harpoon ships also quickly covered their harpoons.
Captain of the MY Steve Irwin, Wyanda Lublink, says the whalers are aware of their actions.
"The fact that the Japanese crew went to cover up their harpoons and the dead minke whale on deck just shows that they know what they're doing is wrong.
"They know they are in contempt of the ruling of the International Court of Justice and the Australian Federal Court. How can the Australian government ignore these actions when the majority of Australians condemn what they are doing?"
The discovery comes just days after Japan Prime Minister Shinzo Abe's official state visit to Australia.
Mr Meyerson says authorities are not doing enough.
"The government has no spine when it comes to protecting the wishes of Australians to defend the Southern Ocean Whale Sanctuary."