Air conditioning has been blamed for a fire that led to the evacuation of up to 500 people at a Docklands apartment block in Melbourne last week.
The fire extended from the third floor through to the 21st level causing $5 million worth of damage.
Metropolitan Fire Brigade (MBF) investigators believe the fire started from equipment a resident had "inappropriately" stored behind an air conditioning compressor unit on their balcony on the third floor. It caused the air conditioner to overheat and a fire to spark.
The investigation will also focus on why fire alarms did not go off in certain levels of the 21-storey building to alert residents about to the blaze.
However, the installed sprinkler systems prevented the fire from spreading further internally.
MBF controller Trevor Woodward said the fire was accidental.
"It's very rare that we've had a fire like this that's able to spread vertically so quickly and that will be part of the investigation as well - was it construction or was it something else that allowed the fire to spread?" Woodward said.
"Hopefully there'll be lessons learnt to be put into place and into proposed buildings in the way they're designed."
There were no fatalities or serious injuries.
Pioneer International general manager, George Haydock, said the cause of the fire doesn't really make sense.
"Could someone explain to me how this could happen, doesn't the air-conditioning unit have safety controls?" he asked adding that he looked forward to seeing the final report.