Police fear someone could get killed because of thieves stealing copper wire from electrical installations.
Wire was taken from 10 transformers in Manawatu this week.
Police say the thieves are exposing themselves and others to the risk of electrical shock and are increasing the danger of fire.
Thirty wind turbines in Manawatu were halted in August by the theft of wire.
Police say copper wire thefts can cut the supply of electricity to people's homes, with potentially serious results.
Lines company Powerco is offering a $5000 reward to help solve the case.