Greenpeace says it has exposed an illegal tuna purse seiner, the Queen Evelyn 168, in a pocket of international waters between Papua New Guinea and the Federated States of Micronesia.
It says the Philippines-flagged vessel was about to transfer tuna to her sister vessel and the refrigerated Kenken 888 when the Greenpeace boat Esperanza arrived.
It says the vessels then immediately separated and fled.
Greenpeace Australia Pacific campaigner, Lagi Toribau, who was on board the Esperanza, says transfers of fish at sea are well known to be facilitating pirate fishing around the world.
Mr Toribau says the pockets of international waters between Pacific Island countries are especially prone to pirate activities and should be closed down to all fishing.
He says transfers of fish should only be allowed to happen in port so they can be monitored properly.
The Queen Evelyn 168 is not authorised to undertake any fishing activities in this part of the Pacific, but the other two vessels are allowed to operate in the area.
All vessels were registered to the Philippines.