The Sea Shepherd vessel has intercepted two of the three vessels caught poaching toothfish by the New Zealand navy last month, captain Sid Chakravarty says.
The Sam Simon had found the Yongding and the Kunlun fishing in waters west of the Ross Sea in the Southern Ocean, Mr Chakravarty said.
He ordered the vessels out the region but one then sailed towards the Sam Simon, avoiding a collision by only 10 metres, he said.
The vessels were flying the Equatorial Guinea flag when intercepted but are not registered there.
Navy ship HMNZS Wellington shadowed the vessels last month but left the area after its crew was refused permission to board.