Andrew Geddis explained the law in this post yesterday.
So while Petrobras are engaged in a lawful activity – exercising their rights (actually, privileges) under law – so what? Because it also is lawful to sail a boat on the high seas, and to leap from that boat into the ocean (if you so choose) … the default position in NZ is “unless the law says you can’t, you can”. Which means that, in and of themselves, the actions of Greenpeace and their allies also are an exercise of rights (actually privileges) under law. So what we have here are the exercise of two rights (actually, privileges) under law that have come into conflict. And that’s all without having to talk about “the right to protest” or invoke the New Zealand Bill of Rights Act, or anything else of that nature.
In other words, the protesters are like the kid who says “I’m punching the air, and if you come near me then that’s your fault”. They’re technally not doing anything wrong, they just “happened” to get in the road and disrupt operations.
In the Kiwiblog thread, someone had a great idea which is probably also perfectly legal but would sort the swimmers out.