A Coroner's court has been told that the gas detection system at a coolstore at Tamahere near Hamilton that exploded in April 2008, killing a fireman, was inadequate.
An inquest opened in Hamilton on Monday into the death of senior fire officer Derek Lovell in the explosion.
Department of Labour chief inspector Keith Stewart told the court that a ruptured pipe leaked refrigerant into the plant, and that a gas detector failed to go off, rendering all other safety systems inoperative.
He says ventilation systems were in place, but the fans in the plant room were not rated to operate in a flammable atmosphere and there were no ventilation systems in place in the coolrooms.
As a result, Mr Stewart says, the gas - which is heavier than air - built up at floor level and it wasn't until it reached the ceiling that the smoke detector was activated about four hours later.
He says there were a number of potential ignition sources, but that a spark from a faulty electrical switchboard was likely to have sparked the explosion.
Mr Lovell and three of his colleagues entered the building, noticed the leak, and were leaving when there was an explosion followed by a second larger one.
Mr Lovell died during surgery, and seven other fire officers were seriously injured.
The session opened with a message from Mr Lovell's mother who asked that the proceedings and resulting findings of the inquiry be his legacy so that he could go on helping people after his death.