It said that may have impacted the way trigger action response plans were applied and actioned.

The inspectorate said a number of other preliminary observations may be relevant to the nature and cause of the incident.

Some key reports relating to the mine's ventilation plan, gas alarm system and explosion risk zone controls did not appear to have been reviewed or countersigned by key personnel, as required under the mine's safety and health management system.

"There is evidence that some boreholes located deep within the 9N goaf region were insufficiently sealed, allowing ingress of oxygen into active goaves, with the potential to escalate conditions for spontaneous combustion," the inspectorate said.

"There is evidence to suggest that the gas drainage system was being operated to focus on management of methane instead of the potential spontaneous heating event that was occurring underground.

"There is evidence to suggest the mine did not follow its own procedures relating to major ventilation changes."

On September 1 all coal mine workers were withdrawn from the underground workings at North Goonyella coal mine as a precaution in response to rising methane levels at the longwall.

In the hours that followed, the mine's spontaneous combustion triggers were reached, escalating the level of risk.

"The inspectorate started gathering relevant information in November 2018 and formally commenced its investigation in January 2019, after the site was stabilised," it said.

"The scope of the investigation is to analyse events leading up to this high potential incident, which resulted in the withdrawal of workers."

The first three months of the investigation involved gathering mine documents and requesting further data where gaps were identified.

To date, the inspectorate has reviewed more than 11,370 files, including ventilation records, gas data and the mine's safety and health management system, including relevant TARPS.

A TARP is a tool that sets minimum actions required by mines to respond to defined "triggers" - events that cause deviation from normal mine conditions.

Failure to adhere to TARPs can lead to escalation of the level of risk without adequate controls being implemented.

"At this time, persons of interest have exercised their right under the Coal Mining Safety and Health Act 1999 not to be interviewed by the inspectorate unless compelled by law to do so," the inspectorate said.

"As such, no interviews have been conducted yet.

"The mines inspectorate investigation is ongoing."