Greenpeace has removed the site office of the proposed Ruataniwha Dam and transported it 100km to the office of the Hawke's Bay Regional Council.
The shed-like office, which was at Tikokino, was uplifted using a truck, and dropped off outside the front doors of the council's office, 100km away in Napier.
Greenpeace is opposed to the controversial dam, saying it would lead to more dairy farming and water pollution.
Agriculture campaigner Genevieve Toop said the council needed to scrap the plan, particularly after the gastro bug outbreak in Havelock North.
Greenpeace was helping by returning the site office, she said.
"It's not theft. We have returned the shed to its owner, the Hawke's Bay Regional Council. We certainly have no need for a shed, so it's been returned to the owner."
But the council said it was "perplexed" as to what Greenpeace was hoping to achieve.
It said the office had been now been moved to another site on Guppy Road.
A council spokesman said the police had been notified.
Last month, Greenpeace was also behind 4000 emails rejecting the dam, which were sent to councillors.