A group of protesters on board a Shell-contracted oil-drilling ship in Taranaki are preparing for a third night on board but their water is running low.

The protesters, including TV actor Lucy Lawless, boarded the Arctic-bound Noble Discoverer and scaled the 53m drilling tower on Friday morning, using locks on the access ladder to barricade themselves on the derrick.

The group of six had a cold and uncomfortable night on Saturday with loud music blasting at 3am and spotlights shining on them all night, Greenpeace New Zealand says on its website.

"They're hungry, and water rations are low."

Ms Lawless, who played Xena the Warrior Princes, says she had some dark thoughts before dawn.

"You think is anybody listening? Is anybody out there? Does anybody even care about this like we do? So, you just don't know, but you've got to do what you believe is right."

The group were preparing to spend a third night on the ship and plan to stay as long as they can, a Greenpeace spokeswoman told NZ Newswire on Sunday.

They had taken supplies to last several days.

One member of the group, Ilai Amir, left voluntarily on Saturday.

He had been arrested and charged with unlawfully boarding a ship.

It had been clear to Mr Amir he would be arrested and he had accepted that risk, the spokeswoman said.

It was a risk the other protesters were well aware of, she said.

Greenpeace hopes to stop the ship heading to the Arctic and is calling on Shell to cancel its plans to open up the Arctic to oil drilling.

It fears the impact an oil spill would have on the ecosystem of the remote location.

Shell says it is disappointed by the protest and its planned drilling exploration is environmentally responsible.

Since the occupation began, more than 122,000 people worldwide have emailed Shell asking it to drop its Arctic oil drilling plans, Greenpeace says.

Police say the protesters are breaking the law but will not be removed at this stage for safety reasons.