Anadarko has begun drilling off the coast of Raglan.
Company spokesman Alan Seay confirmed the Texan oil firm's drilling ship Noble Bob Douglas started "spudding the well" at about 2.30am.
"It basically means that the drillship has started to drill the well."
Greenpeace climate campaigner Steve Abel confirmed the protest ship Vega was still in and around the 500-metre exclusion zone around the drilling ship.
"If the drilling has started then they [the crew] will discuss what the next step is," Abel said.
Protest vessels had prevented Anadarko from breaking ground below the West Coast of the North Island until today, with drilling expected to start last week.
Anadarko's licence began on November 15 and ends on February 14 and it is an offence for any vessel to get within 500 metres of the drillship.
Support for the Vega has grown in recent weeks, with around 700 people turning out for a protest at Raglan on Saturday, one of many run on the west coast of the North Island.
The Vega may have cost Texan oil giant Anadarko $5 million since blocking the Noble Bob Douglas a week ago.
Drilling should have started two or three days after the Noble Bob Douglas arrived off the coast of Raglan on November 20 with the ship expected to put its 4600 metre long drill down in 1500 metres of water last Thursday or Friday.
The operation costs Anadarko $1m a day.
Prime Minister John Key said said the Navy could be brought in to deal with the Vega.
"The law-change that we made clarified some of the roles and effectively put in place the exclusion zone," he said on TV3's Firstline this morning.
"Essentially as I understand it, any operation - if one was required - would be run by the police, but I think it's run alongside Maritime New Zealand and they can deploy Navy assets.
"While the Government would be informed of that, we don't make that decision."
Key told RadioLive yesterday the Government had measures up its sleeve to handle the protesters but refused to say whether or not the military would be called to push the Vega outside the exclusion zone.
Anadarko was held jointly liable by a United States court for the Gulf of Mexico spill in 2010 but denied culpability.
"There are people who are genuinely confused by the data and what they're told," Key said. "You can't say there's no risk, but I think that risk is extremely remote."
Vega skipper and Greenpeace executive director Bunny McDiarmid was worried about people making mistakes rather than technology.
Sailing vessel Tiama skipper Henk Haazen, whose yacht is part of the Oil Free Seas Flotilla, said spirits were good around the flotilla. He said the flotilla had plenty of food and water.
"At the moment we are getting a bit more wind compared to what we have had so far, a good sailing breeze. A nice wind is also called good yacht fuel, renewable and free."
Raglan Maori who complained they were not consulted are planning to serve a trespass notice to Anadarko, which they claim is operating in their ancestral fishing grounds.
- Fairfax Media