Greenpeace claims it has caught spies in the act of tracking its staff and supporters and compiling detailed dossiers.
Greenpeace New Zealand executive director Russel Norman told the Herald the surveillance included tracking people in their private lives.
The environmental lobby group has filed a civil suit in the High Court against Thompson & Clark Investigations alleging breach of privacy and seeking an injunction to stop the surveillance.
Nick Thompson, a director of Thompson & Clark, last night said the firm was bemused by Greenpeace's claim and would respond appropriately in due course.
The lobby group claims to have caught two people spying in "a reverse sting" after it was tipped off that the group was under almost daily surveillance, Norman said.
The group had also been leaked documents. Together they had led Greenpeace to believe they have been the target of a sustained campaign for some years.
Norman said the group had evidence that dossiers - including photographs, vehicle registrations and home addresses - had been compiled on dozens of staff and supporters.
"We have discovered that Greenpeace has been subject to systematic highly intrusive investigations by Thompson & Clark [Investigations] and people working for them," Norman said.
"They have been following our staff and volunteers in their private lives as well as their professional lives.
"We took steps to confirm some of the information we received. That involved looking at some of the dossiers compiled by Thompson & Clark. We also put in place a counter-surveillance operation to catch them in the act.
"And we did catch them in the act, chasing us around and taking photographs of [our] people."
A statement of claim alleges that, "since at least 2014, Thompson & Clark has been engaged by Statoil ASA, a multinational oil and gas company with its headquarters in Norway, and Anadarko Petroleum, an American oil and gas company, to provide intelligence about Greenpeace and its operations".
Approached for comment, a staff member of Thompson & Clark said that the directors Gavin Clark and Nicholas Thompson were currently unavailable.
Requests for comment have been sent to Statoil and Anadarko.
The first plaintiff is a Greenpeace employee (not named in the statement of claim), Greenpeace is the second plaintiff. Both are seeking an order halting the surveillance and costs, and the first plaintiff is also seeking damages and interest.
The Herald understands that covert photographs include Norman swimming at a beach with friends.
Norman said Greenpeace did not know the identity of its source.
"I've got people in the office who are frightened," Norman said. "Staff are feeling nervous on their way to work, and now have to check over their shoulder wherever they go.
"We're talking about people being followed home at all hours of the day and night by spies creeping around in darkened cars with long lens cameras."
The sting operation involved Greenpeace staff taking a vehicle towing a boat from Auckland to Hawke's Bay while people placed along the route observed two vehicles following it.
In 1985 French Government agents bombed Greenpeace's flagship, the Rainbow Warrior.
In April, Norman and two other Greenpeace activists were prosecuted after jumping into the sea in front of oil exploration ship Amazon Warrior 50 nautical miles off the Wairarapa Coast. The ship was conducting seismic sounding for Statoil and Chevron.
The group were charged under a 2013 amendment to the Crown Minerals Act put in place to stop protests at sea near oil exploration operations, and carries maximum penalties of 12 months' jail or a $50,000 fine.
The charges were laid by NZ Petroleum and Minerals, a division of the Ministry of Business, Innovation and Employment.
It was reported last month that the trio would be offered diversion, an avenue that would require them to admit to breaking the law.
At the time Norman accused the Government of "prosecuting climate activists and pandering to oil companies".
Thompson & Clark is a security, corporate intelligence and protection agency. Its registered office is in Milford on the North Shore.
The company has previously been accused of spying on community groups. In 2009 it was accused of trying to recruit a Christchurch man to spy on the Save Happy Valley environment group, which opposed Solid Energy's coal production.
And in 2010 it was accused by a media company of planting a tracking device on a car of an animal rights campaigner. In relation to that claim, Gavin Clark reportedly declined to comment on "anything we might do operationally".
Statoil is an energy company focused on oil and gas. The Norwegian government owns 67 per cent. It began operations in New Zealand in 2013 and has four permits for exploration off the east coast of the Lower North Island.
Statoil was founded in 1972 and operates in 35 countries.
Anadarko has been actively exploring in New Zealand since 2008. It has drilled wells off Taranaki and Canterbury. It opened an office in Wellington in 2011 to focus on stakeholder engagement and community relations.