Japanese Prime Minister Shinzo Abe has been personally handed a letter from Sea Shepherd founder Paul Watson warning that any resumption of whaling will be vigorously challenged.
The International Court of Justice (ICJ) in April ruled that Japan's whaling campaign in the Southern Ocean was a commercial venture and had no research value, which Tokyo said justified the hunt.
Japan called off its 2014-15 Antarctic season but Abe has since signalled that he intends to look at ways it can be resumed without breaching the ICJ ban.
Watson, who has spearheaded a decade of sometimes violent high seas opposition to the slaughter, said his letter was handed to Abe at a state dinner in Canberra on Tuesday by Australian Greens senator Peter Whish-Wilson.
The letter warns that Sea Shepherd ships would resume their harassment of Japanese whalers if another hunt went ahead, while calling on Abe to turn Japan into "a great maritime conservation nation".
"On behalf of the Sea Shepherd volunteers worldwide I would like to respectfully request that Japan abide by the ICJ ruling and that Japan respects both the moratorium on commercial whaling and the Southern Ocean Whale Sanctuary," the letter read.
"If the Japanese whaling fleet returns to the Southern Ocean Whale Sanctuary in 2015, the ships and the volunteers from Sea Shepherd will also return to oppose any continued unlawful whaling activities."
Japanese authorities have previously described methods used by Sea Shepherd against whaling ships -- such as blocking the boats' propellers -- as "terrorist" and said they wanted to arrest Watson.
The Canadian marine conservation group founder said "there is no need for this conflict to continue".
"Japan has no need to kill whales any longer. In fact whaling continues only because of subsidies from the Japanese government," he said.
"In return for ending whaling, Japan will gain the respect and admiration of conservationists around the world.
"I would like to see Japan become a great maritime conservation nation and to shed its present reputation as a nation that takes from the sea and gives nothing back," Watson added.
"Our ocean is dying Mr. Prime Minister and when the ocean dies, we will follow. We cannot live on this planet with a dead ocean."
New Zealand and Australia were the nations that took Japan to the ICJ but both of them this week said their opposition to whaling would not affect the bilateral relationships.
The commercial hunting of whales is prohibited in the Southern Ocean Whale Sanctuary, which was designated by the International Whaling Commission in 1994.
However, Japan had been hunting the animals there under a "scientific research" loophole until the court's decision.