Alarm over the biggest ship ever planned to fish Australian waters has spilled into direct action, with the supertrawler Margiris locked down by Greenpeace.

Climbers and divers are preventing the 9600-tonne ship from leaving the Dutch port of Ijmuiden to begin the journey to Australia, the environment organisation said.

The supertrawler Margiris is locked down by Greenpeace activists in the Dutch port of Ijmuiden today. Credit:Greenpeace

"At 0300 AEST activists put a chain around the ship's propeller, and two climbers are currently hanging on the cables between the ship and the quay, to prevent the ship from beginning its journey to Tasmania," Greenpeace said.

The Dutch-owned ship is being reflagged to Australia before its scheduled introduction within weeks, according to joint venturers Seafish Tasmania.