Sydney's Downing Centre Local Court Magistrate Ross Pogson imposed the maximum penalty but said it was inadequate.

The eight were part of a Greenpeace protest in which two activists clambered up the sides of the frigate HMAS Sydney and unfurled a "no-war" banner in a demonstration that brought the vessel to a halt on April 9.

The two managed to attach themselves to the hull of the warship with climbing equipment, embarrassing Australian authorities.

All eight, Geoffrey Nimmo, Nicholas Clyde, Megan Ivory, Jan Madsen, Logan Petley, Sarah Roberts, Philip Wollam and Michael Rosato, pleaded guilty to wilfully preventing the free passage of a vessel.

Nimmo read a statement to the court on behalf of the group saying he and some of the others had served in the armed forces and the action had targeted the government, not the troops.

"Our action was based on respect for international law over and above unilateral action," Nimmo told the court.

Defence barrister Grant Brady said all activists were full-time workers trained in either climbing or diving so there was no safety risk in the protest.

"Each and every defendant believed firmly ... that the actions of the Australian government in sending troops to Iraq was an action that was without UN sanction and therefore illegal," Brady said.

Magistrate Pogson said the protesters may well have had significant community support for their cause but their actions would have been offensive to any fair-minded person.

He said the maximum penalty was inadequate because it did not convey the seriousness of the offence.

SPACE.WIRE