"At this time, all causes have been eliminated with the exception of electrical infrastructure or possible arson," he said.

Mr Millington said the investigation, which was completed on Tuesday, also found the fire had started outside the Telstra pit.

"The fire investigator conducted a thorough scene examination, and has spoken with residents, Telstra workers, as well as electricity workers as part of the investigation," he said.

"A copy of the report has been provided to NSW Police Force, which may be used as part of any broader investigation."

While the cause of the fire was undetermined, Mr Millington said there was "no lightning in the area at the time".

'That certainly has been ruled out following an analysis of data and information provided by the Bureau of Meteorology," he said.

The outage is currently under formal investigation by the government, which has a multimillion-dollar contract with Telstra to provide the emergency line. The Australian Communications and Media Authority is assisting with the line of inquiry, including seeking information about how many calls were affected and the failure of the telco's backup systems.

At the time of the outage, a Telstra spokesman said the observation from the team on the ground was "fire damage consistent with a lightning strike" but confirmed the telco's team was working with the Country Fire Authority and local groups to help determine the actual cause.

Telstra's main focus in the immediate days following was why the network backup did not respond as planned, the spokesman said, rather than the cause of the cable damage.

Telstra has been contacted for comment.