The activists were protesting Straterra's plans to fast-track a seabed mining project for one of its clients, Trans-Tasman Resources, in the South Taranaki Bight.
There were two protesters on a ledge on the outside of the building on The Terrace on Tuesday morning, and at least one other was inside.
There were two police cars, a van, and two fire trucks, and the area around the building had been cordoned off for health and safety.
About 30 people were watching from the street, some of them unable to enter their workplaces.
In a statement Greenpeace said it was "a demonstration of the resistance promised" in a recent open letter to companies considering using the fast-track process, which aims to speed up infrastructure consenting.
More than 7500 people had co-signed the letter, Greenpeace said.
The group called Straterra a "malignant force in New Zealand politics, operating in the shadows and backrooms to exert a pernicious influence over Government policies".
Trans-Tasman Resources declined to comment. RNZ has also approached Straterra.
