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.