The RFS backburn which eventually claimed homes in and around the Blue Mountains in December 2019 was brought forward to avoid forecast catastrophic weather, an inquiry heard last week.
But in the end, conditions on December 14, the day it was lit, deteriorated, causing spotovers to spread rapidly and take out numerous properties in areas the RFS was trying to protect, counsel assisting the coronial inquiry, Adam Casselden SC, said.
"Twenty-two homes and 30 outbuildings were decimated [and] a number of harrowing incidents resulted in various injuries to those responding [to the fire] and residents."
Mr Casselden said in November 2019, the Gospers Mountains fire, which had started on October 26, was out of control and headed towards the Mountains.
By December 9, the RFS was facing a 60-kilometre wide fire front that threatened to encroach on townships to the south of Bells Line of Road.
Two days later, the incident management team decided a backburn should be lit on the evening of December 15. But after a new forecast from the Bureau of Meteorology predicted worse conditions on December 16 and a slight improvement for December 14, the decision was made to light the backburn on December 14 instead.
The burn was lit at the intersection of Mt Wilson Road and Bells Line of Road before 10am but as the afternoon wore on, the wind direction changed to the south-west and humidity dropped.
The day, Mr Casselden said, became "warmer, drier and windier than forecast". Crews were instructed to stop lighting and requests were made for retardant to be dropped.
The fire spotted on the other side of Mt Wilson Road and over the Bells Line of Road into the Grose Valley. By the following day it fire had spread to Mt Wilson, Mt Tomah, Berambing and Bilpin, where properties were lost.
It would later also destroy homes in Lithgow, Clarence, Bell, Dargan, Bilpin and Kurrajong.
Mr Casselden said he was aware of "concerns around the backburning and also issues of communication with communities", but he noted that this particular part of the inquiry was concerned only with the cause and origin of the Grose Valley and the Gospers Mountain fires.
The reasons behind the backburn strategy may be canvassed at further hearings in September, he said.
Mr Casselden also told the coroner, Teresa O'Sullivan, it was most likely the Gospers Mountain fire had been started by a lightning strike deep in the remote Wollemi. It eventually became the largest single fire in Australia's recorded history, burning out 512,626 hectares.
The hearings are continuing this week.
