Seven Greenpeace members have boarded an oil drilling ship at Port Taranaki to protest its involvement in oil and gas exploration in the Arctic.
The protesters boarded the Noble Discoverer at 7am and climbed to the drilling tower to stop it sailing.
The ship has been contracted by Shell Oil to drill three exploratory wells in the Chukchi Peninsula off the coast of Alaska.
Police say after meeting with the ship's owner and the port they have agreed to leave the protesters where they are on Friday night.
Police say even though they have the skills and equipment to remove the protesters, they are not yet prepared to jeopardise anyone's safety by using force, and will monitor the situation.
Greenpeace says the protesters, which include actress Lucy Lawless, are well equipped with sleeping bags and food and will stay on board for as long as they can.
Earlier, police said one person had been arrested.
Shell Oil says actions such as this jeopardise the safety of everyone involved, and while it respects the right of the protesters to express their point of view, the priority should be safety.
The company says it has taken steps to pursue environmentally responsible exploration off Alaska.