Runanga deputy fire officer Aaron Sheehan said the on-site foreman raised the alarm when smoke set off the alarm system.
However, prior to their arrival, the fire brigade was notified the fire was out, the area was isolated and the smoke cleared.
One Greymouth and two Cobden appliances returned to station, while remaining Runanga and Greymouth crews checked over the scene with thermal imaging equipment, and were greeted by a couple of rats that had moved into the meter box.
Mr Sheehan said one had got between two wires, causing a short circuit.
"Fortunately, there were electricians on site who shut the power down", and removed the frizzled rat.
Mr Sheehan said the short circuit caused a fair bit of smoke which was ventilated out of the building.
It is thought the affected switch may take several days to replace, leaving the building without power in the meantime.
- Meg Fulford
