Greenpeace claims it has caught spies in the act of surveilling staff and supporters.
The environmental group's executive director Dr Russel Norman claims a spy agency had people following staff home at night, tracking their movements, photographing and videoing them in their private lives.
Dr Norman says a spy agency called Thompson and Clark is responsible for the "highly intrusive" operation and has allegedly been working with large multinational oil companies over a number of years.
"We caught the spies in the act and recorded what they were doing," he said. "It's a very expensive operation that's been going on for five years or more."
He told Newshub Greenpeace has filed a civil lawsuit alleging breach of privacy to try and get an order for the surveillance to end.
"In a free and democratic society, people should be allowed to engage in protest and climate activism without oil companies and large spy agencies engaging in this kind of intrusive activity."
Dr Norman believes the spying operation has stemmed from Greenpeace's fight against climate change.
"Greenpeace is trying to stop the expansion of the oil industry, and the oil industry is hiring private investigators to engage in surveillance into our activities, [as well as] our staff's private lives, as a way to strike back against the climate movement."
He says Greenpeace is always extremely transparent about its actions and only engages in non-violent, peaceful protest.
"This is really intimidating and extremely unsettling, and it makes it very difficult to operate in a free and open way."
Newshub has contacted Thompson and Clark for comment. The agency told NZME it was bemused by Greenpeace's claims and would respond in due course.