The environmental activist group, Sea Shepherd, says its ship was charged at by a vessel poaching toothfish in the Indian ocean last night.
The group's Bob Barker ship has been pursuing the Nigerian-flagged Thunder for more than 50 days.
Bob Barker captain, Peter Hammarstedt, said he found Thunder trying to pull up an illegal net, so positioned his vessel to block it.
He said his ship was then charged at and the boats were half a metre away from a collision.
Mr Hammarstedt said Sea Shepherd is having to do work that should be done by the New Zealand navy.
He said the seven week pursuit has probably cost the Thunder $20 million in lost revenue, and the poachers are getting more and more frustrated.
Sea Shepherd said earlier this week, its other ship, Sam Simon, intercepted two other illegal fishing vessels in the Southern Ocean.