The Office of NSW Transport Safety Investigations was due to release its report with the findings this month into the derailment of the Pacific National train which derailed in February, last year.

But now the report has been delayed by four months as it undergoes a “process of review”.

The Pacific National train was en route to Narrabri for loading when 19 empty wagons derailed just after 5am on February 15, 2015, at Kankool, near Willow Tree. 

On the morning of the derailment, the crew of the NB901 coal service contacted the Network Control Centre North at Broadmeadow, reporting their train had lost air and come to a stop. 

The assistant driver went to investigate the loss of air and discovered the leading 19 wagons had derailed, causing severe damage to the train track.

The incident saw the line blocked for five days and a Senate Estimates hearing was told the damage bill for the Australian Rail Track Corporation (ARTC) was around $1 million. 

The investigation into the cause of the derailment is being led by the Office of NSW Transport Safety Investigations and is being overseen by the Australian Transport and Safety Bureau who had initially anticipated to release the report at the end of 2015.

But the report, which will be examined by key stakeholders before it is made public, was then expected to be released in May, but on Friday, investigators confirmed to Fairfax Media it was undergoing a “peer review” and would be delayed.

Investigators now expect to release the report in September.