A Norwegian firm which was until recently prospecting for oil in Northland says it didn't hire private investigators to spy on Greenpeace - but it does pay a firm to "ensure the safety" of its personnel.

Last week environmental organisation Greenpeace revealed it had used a "reverse sting operation" to catch out private investigation firm Thompson & Clark as its staff followed a Greenpeace vehicle towing a boat from Auckland to Hawke's Bay.

Greenpeace NZ executive director Russel Norman said he had evidence Thompson & Clark had also compiled detailed dossiers on Greenpeace staff and supporters, and had tracked them in their private lives since at least 2014.

Greenpeace, which is taking legal action against the alleged surveillance, claims it is being done on behalf of Statoil, a state-owned Norwegian oil company, and Anadarko, a US oil firm.

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Statoil won a tender to explore the Reinga Basin off Northland's west coast but pulled out in 2016, saying its seismic survey data showed little chance of striking gas or oil. The company is still exploring elsewhere in New Zealand waters.

Statoil's New Zealand country manager Brynjulv Klove told the Advocate his company had not contracted Thompson & Clark, or any company, to conduct surveillance of Greenpeace or its members.

"However, we are a client of Thompson & Clark. They provide advice and services relevant to ensuring the safety and security of our personnel," Mr Klove said.

Mr Klove said he would not comment further because it involved a legal process that Statoil was not a party to and which was under way in the New Zealand court system.

Nick Thompson, a director of Thompson & Clark, said the firm was bemused by Greenpeace's claim and would respond appropriately in due course.

In 1985 spies from the French Secret Service bombed the Greenpeace vessel Rainbow Warrior in Auckland Harbour, killing Fernando Pereira, a Dutch citizen of Portuguese origin.

Statoil was granted a 15-year permit to explore the 10,000sq km Reinga Basin, about 100km off Ninety Mile Beach, in December 2013. It pulled out in October 2016.