"While the driver was outside of the locomotive, the train commenced to runaway", the ATSB statement said.
The train was made up of four locomotives and 268 wagons. The engineer had left the locomotive to inspect an issue with an ore auto, and the train then began moving. It was eventually deliberately derailed, which was operated by a control center.
BHP Billiton has suspended all iron ore rail operations in Western Australia after a runaway freight train with no one on board travelled 92km before it had to be deliberately derailed.
The miner suspended all of its rail operations on Monday after it derailed the iron ore train, damaging 1.5 kilometres of track and crushing numerous 268 fully-laden wagons in the process.
The ATSB is investigating the incident and expects its report will be complete in the second quarter of 2019. It averaged about 68 miles per hour and traveled for some 50 minutes before it was derailed, according to Australia's ABC.
The interruption to the train line could be particularly costly for the mining company - given that it's the main way to deliver ore from its Pilbara mines to Port Hedland. While the driver was outside the train, it took off with no one on board.
"However, we are working with the appropriate authorities and our focus remains on the safe recovery of our operations".
"Usually, once the driver leaves the train, the brakes are on, there's procedures for that", Rail Safety Consulting Australia owner Phillip Barker told ABC.
The derailment came after the train ran away at high speed for almost 100 kilometres (62 miles).
There has been no disruption so far to shipments of BHP's medium-grade iron ore cargoes to top market China, traders said.
In July rival mining giant Rio Tinto clocked up a world first when its maiden driverless train voyage carried 28,000 tonnes of iron ore 280 kilometres from its Mount Tom Price mine to a WA port.