The Employment Relations Authority has ruled the Mighty River Power plant at Kawerau is a safety sensitive area, but it has not decided yet if personnel there can be forced to undergo random drug tests.
The ERA has ruled on the case of Dean Cowell who refused a test and was stood down from work.
Mighty River Power told employees in August that as a result of a number of incidents, it had amended its drug and alcohol policy to allow for random testing in safety sensitive areas.
The Electrical Union argued the changes were a breach of the collective employment agreement and were unlawful and unreasonable.
But the Employment Relations Authority says the power plant is a safety sensitive area and the consequences of an accident at the site could be catastrophic.
The authority will consider the wider issue of the legality of random drug testing on 29 November.