NSW Rural Fire Service Chief Superintendent Ben Millington, who has oversight of fire investigations, said a "comprehensive and thorough" investigation into the fire was unable to determine a cause.

"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.

A Telstra spokesman said the telco operates "millions of kilometres of fibre cables around Australia and these are subject to a number of risks of damage every day, including cuts by construction work, weather, vandalism and fire".

"These risks are top of mind when we design our network and we build in redundancy for key services and routes in case of a cable cut," he said.

He confirmed Telstra would consider the NSW Rural Fire Service's findings as part of its review and response to the incident.

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.

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.