Two expert witnesses will give evidence to the UN's top court to try to convince a 16-judge panel that Japan's whaling activities are commercial not scientific.

The International Court of Justice (ICJ) doesn't usually hear from experts but US mathematics professor Marc Mangel and Australian Antarctic scientist Nick Gales will both appear at The Hague on Thursday for day two of the three-week hearing.

Australia is hoping they'll help prove Japan's JARPA II research program is not based on science.

Philippe Sands QC on Wednesday pointedly noted there was no such "research" conducted before Japan agreed to the commercial whaling ban in 1986.

"The supposed scientific research emerged as a happy coincidence when the moratorium came into view," he told the court on day one.

"There is no independent body that supports Japan's claim to be engaged in science."

Canberra also argues Tokyo can't write its own leave pass under Article 8 of the 1946 whaling convention which allows harpooning "for purposes of scientific research".

"An indeterminately broad scientific research exception, that could be invoked and defined at the discretion of one or indeed all of the 89 parties, would entirely defeat the convention regime," James Crawford SC said.

"It is ultimately for this court to decide whether a whaling program does or does not fall within Article 8."

Lead counsel Bill Campbell QC argued if every convention country killed as many whales as Japan wanted to each year more than 83,000 minke whales would be harpooned annually.

That would be "catastrophic" for the whale population and at odds with the convention's purpose to conserve stocks.

Mr Campbell later told reporters Australia had laid the groundwork "for the court ultimately seeing the case our way".

"There is a difference between ourselves and Japan in relation to what is and what is not science," he said.

"The evidence from the witnesses (on Thursday) will be important for that."

The court usually hears from government representatives and lawyers only.

Japan on Thursday will be able to cross-examine both Professor Mangel and Dr Gales.

Government spokesman Noriyuki Shikat says Tokyo remains "very confident".

"As far as the international legal argument is concerned we are talking about Article 8 which in a very straightforward manner accepts special permit whaling," he told reporters at the Peace Palace.