According to Associate Professor Natalie Klein of Macquarie University, the actions of New Zealand Sea Shepherd activist Peter Bethune, who was arrested in February after boarding Japanese whaling vessel Shonan Meru 2, could be constituted as piracy.

The issue was discussed at a recent forum titled Pirates of the Southern Oceans, held last week at the Australian National University.

Bethune has been indicted by Japanese authorities after he approached the Shonan Meru 2 with a jet ski, boarded without permission, and presented the captain with a bill for $3 million in damages sustained during a collision between the Shonan Meru 2 and the Sea Shepherd's Ady Gil in January.

"Professor Klein's conclusion was that you could make an argument that Bethune, by his actions - especially boarding the ship and making a demand of the master - could have been considered as demanding money by way of violent means," said forum organiser Professor Donald Rothwell.

"Technically, the various boxes have been ticked in relation to piracy, but Professor Klein's comments were directed towards Bethune, as opposed to the Sea Shepherd Conservation Society generally."

Japanese authorities have declined to prosecute Bethune under international laws pertaining to piracy, and have instead charged him under domestic laws including trespass, assault, destruction of property, possession of a knife and obstruction of business.