The forced derailment of a fully-laden iron ore train has inflicted a heavy financial blow on BHP, with the incident contributing to a $US600 million hit to the miner's productivity.

Key points: The train was carrying 30,000 tonnes of iron ore when it took off without the driver

The train was carrying 30,000 tonnes of iron ore when it took off without the driver The fully-laden train, pulling 268 wagons or 30,000 tonnes of iron ore, was travelling from Newman to Port Hedland

The fully-laden train, pulling 268 wagons or 30,000 tonnes of iron ore, was travelling from Newman to Port Hedland The driver was sacked, with an unfair dismissal claim believed to be underway

Last November's derailment — which left the train spectacularly wrecked in a remote part of WA's Pilbara region — hit BHP's iron ore production by four mega tonnes, and was by far the biggest factor in the overall lower-than-expected volumes in the December quarter.

That meant the derailment cost the company hundreds of millions of dollars, hitting BHP's iron ore productivity by six per cent in the final three months of 2018.

"Productivity for the December 2018 half year has been impacted by unplanned production outages at Olympic Dam, Spence and (Western Australia Iron Ore), with a total negative impact of approximately $US600 million," BHP's half-yearly operational review stated.

"During the rail outage, mine stockpile levels were built and are expected to be partially drawn down in the March 2019 quarter."

The train had to be derailed after taking off without the driver. ( Supplied )

The fully-laden train, pulling 268 wagons or 30,000 tonnes of iron ore, was travelling from Newman to Port Hedland when the driver got out to inspect a wagon near Hester siding.

While the driver was out of the cabin, the train took off, travelling more than 90 kilometres in 50 minutes.

The train, made up of four locomotives, was eventually derailed at a set of points at Turner's siding, about 120km south of Port Hedland.

The derailment, which left a twisted and mangled wreckage, was orchestrated from BHP's Integrated Remote Operations Centre in Perth, about 1,500km away.

The train was pulling 268 wagons loaded with 30,000 tonnes of iron ore when it was derailed. ( Supplied )

All but 24 of the wagons of the train were damaged by the derailment, along with two of the four locomotives.

The driver of the train was later sacked by BHP, with an unfair dismissal claim understood to have been lodged.

Investigations into the derailment by the ATSB and the Office of the National Rail Safety Regulator are continuing.