Greenpeace says the rest of the protesters have vowed to fight on and will spend a second night on the tower.
The group hope to stop the ship heading to the Arctic and are calling on Shell to cancel its plans to open up the Arctic to oil drilling.
Greenpeace said since they began their occupation more than 96,000 people had emailed Shell in support of the cause.
Shell New Zealand chair Rob Jager said the company was disappointed by the occupation, which jeopardised the safety of all involved, including the crew, and would happily talk to Greenpeace.
Shell had spent several years developing plans to explore off the coast of Alaska safely in an environmentally responsible way, he said.
Lucy Lawless says: "The fact is that if drilling is allowed to go ahead it's not a question of if there will be an oil spill in the Arctic, it's a question of when."
New Plymouth area commander Blair Telford said the protesters were "clearly breaking the law" but were in an isolated location on board which allowed both the port and the ship to operate normal business.
"We have considered various options and we have the skills, capabilities and equipment to deal with all options. However safety remains paramount and we won't needlessly jeopardise the safety of our staff, the crew of the ship or the protesters."
XENA ECO-WARRIOR
Lucy Lawless says she has accepted the prospect of criminal charges and potential career damage after illegally boarding a ship to protest against oil drilling in the Arctic Ocean.
The Xena: Warrior Princess star sneaked onto the Shell-contracted Noble Discoverer with six other Greenpeace protesters at Port Taranaki at dawn yesterday, climbing its 53-metre drilling tower and hoisting banners which read "Stop Shell" and "Save the Arctic".
The group are attempting to prevent the Liberian-flagged ship from drilling three exploratory oil wells in the remote and ecologically fragile Chukchi Sea off the coast of Alaska. The drill ship is due to leave for the region via Seattle this weekend.
Speaking to the Weekend Herald from the "uncomfortable" peak of the ship's tower, Lawless said: "I'm a mother, I don't do this stuff. But I feel I have to stand up and be counted and act on behalf of my children and grandchildren."
Greenpeace is particularly upset that Shell is capitalising on ice melt in the region to seek more oil reserves.
Lawless said: "To see the melting of the sea ice not as a warning to humanity but as an invitation to drill for more of the stuff that caused the problem in the first place is the definition of madness. What Shell is doing is climate-change profiteering."
Lawless noted that her stunt-training for Xena had prepared her for the task of climbing the ship's tower.
"I feel like I have been training for this all my life. We will be here for as long as it takes."
Asked whether criminal charges could threaten her future acting projects, she said: "It could. But I can only be myself."
Shell's oil spill response plan for the expedition was approved by United States environmental safety authorities last month. But Greenpeace climate campaigner Steve Abel argued that its response methods were untested in Arctic waters.
GNS Science exploration geophysicist Chris Uruski said that since the Gulf of Mexico disaster oil giants were acutely aware of the consequences of a major oil spill near fragile ecosystems.
"I think that the fact that a spill is enough to endanger the very existence of BP - and it's not exactly a small company - that's incentive enough to make sure that those things don't happen."
Shell said its commitment to basic principles of environmental safety was "unwavering". A request for further comment from Shell New Zealand was not replied to.
The Noble Discoverer has a 105-day window for drilling while the ice retreats, but it has to stop drilling after 67 days to ensure it has time to fix a well blowout.
