The Tararua wind farm, which is owned by electricity retailer TrustPower and located near Palmerston North, was broken into in the early hours of August 16.

The thieves braved 220,000 volt transformers and drained the coolant oil, causing the turbines to cease operating. It is estimated around 20,000 litres of cooling oil was lost from two transformers.

Taraua was the largest wind farm in the southern hemisphere when it was constructed in 1999. It consists of 103 V47 660KW and 31 V90 3MW turbines.

Speaking to NZPA, a TrustPower spokesman Graeme Purches ruled out the possibility the damage was caused by wind farm protestors.

Commenting on the unlikelihood he said protestors "generally have a skerrick of intelligence" and would have avoided the high voltage area of the development if they were seeking to cause damage.

Purches added: "Whoever did this, they must have been seriously seeking to raise the IQ of the whole nation.

"People don't go into switch yards. Switch yards are very dangerous places, there's high voltage, there's magnetic fields, everything. That sort of voltage travels long distances if you look like you are a likely conductor.

"Not only had they dug up copper cables, not knowing what they were doing, but they drained the oil from two transformers. We are talking 20,000 litres of cooling oil."