Rural Fire Service fire investigation training coordinator, Mark Fullagar, said there were 130 authorised fire investigators in the organisation during the catastrophic 2019-20 bushfire season.
But only 10 of those investigators, many of whom were volunteers, were available at any one time.
"Considering the season we had, it didn't surprise us," Mr Fullagar told the long-running coronial inquiry, sitting at Lidcombe Coroners Court on Tuesday.
"Just the amount of fire that was on the ground and staff and volunteers being used in other roles.
"Volunteer fire investigators have roles within their brigades as firefighters."
Mr Fullagar agreed with counsel assisting Donna Ward SC's assertion that many fires were not investigated because RFS resources didn't stretch far enough.
Since Black Summer, there are even fewer authorised bushfire investigators - 60 RFS volunteers and 46 staff.
Mr Fullagar said the decrease might be attributed to Black Summer, retirements, a change of roles and potentially COVID-19.
RFS Assistant Commissioner Ben Millington said the fire investigation unit has increased the number of dedicated fire investigation officers and is future-proofing its workforce as fire seasons get longer and more complex.
Detective Sergeant Laura Harvey, the officer in charge of the investigation into the Grose Valley Fire at Mount Wilson in the Blue Mountains, told the inquiry she requested a cause and origin investigation near the Bells Line of Road.
One month later, she discovered a fire investigation had instead been undertaken at the Grose River to understand how the fire moved between two local government areas.
Det Sgt Harvey agreed she was frustrated by the change of location, and would have said so if she'd been informed earlier.
"It's not helpful as to the cause and origin of the fire," she said.
Investigations later determined the fire was caused by a "spot over" from back burning, designed to protect the Mount Wilson community.
The inquiry continues.
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