At the same time, the New Zealand Government needed to take "a very forceful" stance and insist the Japanese co-operate with Maritime New Zealand's investigation of the collision between Bethune's vessel, Ady Gil, and the Japanese whaling fleet's security ship Shonan Maru II in January, Prof Rothwell said in an interview at the University of Otago this week.
"It's a significant diplomatic issue."
Bethune, a New Zealand anti-whaling activist, went on trial in the Tokyo District Court on Thursday after boarding Shonan Maru II following the sinking of the Ady Gil.
Bethune denies injuring a Japanese crew member and injuring a Japanese whaler with a rancid butter, or butyric acid, stink bomb, but has admitted obstructing business, trespass, property destruction and violation of the weapons control law by carrying a knife with a longer-than-legal blade.
A verdict is expected next month.
The professor of international law at the Australian National University College of Law has been touring New Zealand universities this week to speak about international law options to halt Japanese whaling in the Southern Ocean and the clashes between the Sea Shepherd Conservation Society and the Japanese whaling fleet.
He said New Zealand should look to Australia's experience in dealing with difficult diplomatic cases. It had shown a country could be "tough as possible" on one issue while recognising other areas of a relationship were going quite well, he said.
"The trick will be to quarantine the issue ... but not let it impact on other aspects."
Prof Rothwell's impression from his time in this country was that New Zealanders were concerned the Government was not taking as strong a stance on those issues as expected.
However, he believed New Zealand, given its role in maritime safety in the Southern Ocean and that the collision involved a New Zealand ship and citizens, should expect more from Japan in its co-operation over the sinking.
"I'm perplexed as to why New Zealand has not adopted a stronger stance on the issue of co-operation by Japan in this matter."
Bethune's trial was "unhelpful" to the larger debate about Japanese whaling in the Southern Ocean because it "clouded the debate". The International Whaling Commission starts talks this weekend in Morocco on a proposal about the future of Japanese whaling, Prof Rothwell said.
There were concerns about the plight of a New Zealander in a foreign country - a situation which some believed was unwarranted - while in Japan, it had provoked debate and support for the Japanese Government and its whaling programme.
"It probably creates some difficulties for the New Zealand Government that they could have done without."
He was "very pessimistic" that the reform proposal being discussed by the whaling commission would be adopted, given there was little middle ground between those countries which supported whaling and those that did not.
While Australia was quite firm in its lack of support for the proposal, the New Zealand Government had seemed to "backtrack a little" on its original stance of supporting the proposal, probably due to the "fierce reaction within New Zealand to support for the resumption of commercial whaling", he said.
Depending on the commission's decision, there would be three options for New Zealand to consider: taking the case to the International Court of Justice with Australia, using the mechanisms of the Antarctic Treaty, or challenging Japan's interpretation of the Southern Ocean whale sanctuary's ban on commercial whaling, Prof Rothwell said.
