Prosecutions against those behind fishing vessels that tangled with the New Zealand navy have been hailed as a "giant step forward" by Foreign Minister Murray McCully.

Spanish authorities will bring prosecutions against against those responsible for fishing vessels - the Kunlun, Yongding, Songhua and Tiantai - associated with the Vidal Armadores syndicate.

"While it is too early to declare victory, these prosecutions are a major step forward and the product of unprecedented levels of cooperation between a range of Governments since January," Mr McCully said.

"Five of the seven vessels that have regularly fished in the Southern Ocean over the past five years have been detained this year, and a further one has sunk."

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In January, HMNZS Wellington docked in Dunedin after eight weeks patrolling the seas around Antarctica.

The last two weeks of the patrol were spent pursuing the Songhua, Kunlun and Yongding, that were fishing illegally.

The standoff ended when the navy was unable to board the foreign-flagged ships and began running low on fuel.

Lieutenant commander Graham MacLean says plenty of evidence has been collected on illegal fishing activities. Photo / Linda Robertson
Lieutenant commander Graham MacLean says plenty of evidence has been collected on illegal fishing activities. Photo / Linda Robertson

Opposition MPs described the operation as ineffective and demanded answers from the Government about how three apparently run-down fishing boats outlasted the navy's offshore patrol vessel.

The Wellington's commanding officer, Lieutenant Commander Graham MacLean, defended the decision to abandon attempts to board the ships, saying enough evidence of illegal activity had been collected and conditions had become too challenging.

Photographs released by the Defence Force showed fishermen hauling in toothfish using gillnets, which are banned in the region.