Australia has argued that Japan's whaling programme is untenable and dangerous in its opening address to the International Court of Justice in the Hague.
The Australian Government launched the legal action in 2010 and both sides are making submissions over the three weeks set down for the case.
The ABC reports there will be no appeal when the court releases its decision later this year, so the stakes are high for all parties.
Bill Campbell, QC, told the court on Wednesday that Japan erroneously asserts the programme is for scientific research.
"Japan seeks to cloak its commercial whaling under the lab coat of science. It simply isn't science," he said. "What Japan is doing in the Southern Ocean is patently for commercial purposes.
"The amount of catch they're taking, which in the case of minke whales, they can take up to 935 minke whales a year. They also sell the product into various places in Japan."
Mr Campbell told the 16 judges the court has a genuine opportunity to decide what does and does not constitute scientific activity.
He said that if each of the 89 countries signed up to whaling regulations made the same decisions as Japan, the consequences would be disastrous.
The ABC reports Australia says more than 10,000 whales have been killed since 1988 as a result of Japan's programs.
It says that puts Japan in breach of international conventions and its obligation to preserve marine mammals and their environment.
Australia's address will last for three days, with Japan making its first submission next week.
New Zealand is considered an intervener in the case, which means it backs the Australian argument and is bound by the court's decision but does not bear the same responsibilities or costs. It will also be allowed to make a brief statement to the court in July.
The court usually takes six to nine months to make its rulings.