Japan says it has been painted as a rogue state by its so-called friend Australia over Antarctic scientific whaling.
In a strenuous defence, counsel for Japan, Payam Akhavan, said it stood before the International Court of Justice answering baseless allegations of bad faith.
Australia has told the court Japan abused the global whaling treaty's article on scientific whaling to carry out commercial whaling, cloaked in the lab coat of science.
More than 10,000 whales have been killed under the program.
Professor Akhavan, of Canada's McGill University, said Australia's case was that Japan had lied systematically, as a matter of state policy, for almost 30 years.
''Needless to say, this is a serious accusation, an affront to the dignity of a nation,'' he told the court, which was to hear Japan's final argument late on Tuesday in The Hague.
Professor Akhavan said Japan had decided that, rather than ending the harassment, intolerance and insults by leaving the International Whaling Commission, it would stay because of its genuine commitment to the rule of law and peaceful dispute settlement.
''Australia comes before this court to take advantage of that commitment,'' he said. ''To unfairly and unreasonably portray Japan as a rogue state at the IWC; to level accusations of bad faith against what it deems to be a friendly state that it can mistreat with impunity.''
He said even while the case was under way, Attorney-General Mark Dreyfus was giving interviews saying Australia would continue its campaign if the case was lost.
''The Attorney-General makes clear that [Australia] will persist in harassing and confronting Japan if this court does not give Australia the decision that it wants,'' Professor Akhavan said.
Australia's final arguments were heard last week, when counsel James Crawford called for Japan to be held to account for a program that was not science but random hunting attuned to the Japanese market for whale meat.
Counsel for Japan, Yukiko Takashiba, said Australia claimed Japan thought it could do what it liked under article eight of the whaling treaty. But she said Japan had listened to the voices of other countries by deciding to abstain from catching humpback whales.
The court's decision is expected within four to six months.