Vandals have broken into the Tararua windfarm near Palmerston North, putting 31 turbines out of action in an attack Trustpower says could have cost lives.
Intruders broke into a switchyard at 2am on Monday, draining cooling oil from transformers and digging up underground cables in an apparent attempt to steal copper wire.
Trustpower community relations manager Graeme Purches says it is the most stupid thing he has seen in a long time - if not ever.
Mr Purches says switchyards are incredibly dangerous places dealing with hundreds of thousands of volts and the thieves were lucky to survive.
He says draining the oil from operating transformers beggars belief and could have caused it to erupt in their face.
Trustpower says repairs will take until late Wednesday at the earliest, costing the company $446,400 in lost generation, assuming the wind was blowing for the whole period.
Te Rere Hau windfarm, owned by NZ Windfarms Limited, shares the same switchyard and faces potential generating losses of $232,000.
Police are investigating but will not reveal how much copper was stolen.
Wind Energy Association chief executive Fraser Clark says copper is in demand and the thieves could have targetted any power transmission and distribution facility. He is urging power companies to check their security.