Benmore project site manager Alan Bulling said the man, Mr Suprihno (42), of Jakarta, was working at Benmore about 10.45am supervising the assembly of a 225MVA transformer shipped from Indonesia.
He was inside the transformer, with the appropriate safety equipment, when he slipped and fell between 1m and 2m.
"The inside of a transformer can be pretty slippery with oil," Mr Bulling said.
Mr Suprihno hit his head, but was able to walk out of the transformer unaided. Mr Suprihno was taken by ambulance to Oamaru Hospital for treatment and observation.
The Department of Labour's occupational safety and health was notified of the accident.
Police, a unit and co-response unit from the Otematata fire brigade and a rescue unit from the Omarama fire brigade attended.
