The fire, known as the Little L Complex, began in late November 2019 and burnt for nine weeks across 171,000 hectares in the Yengo National Park.
It damaged or destroyed up to 30 buildings, including four homes, in the villages of Laguna, Wollombi and Paynes Crossing.
The blaze was not extinguished until January 24, 2020, due to its remote location and the need to direct firefighting planes and helicopters elsewhere, the NSW bushfires coronial inquiry was told on Monday.
Counsel assisting the coroner, Donna Ward SC, said part of the blaze spread across 11 kilometres in one day, considered "extreme and anomalous" fire behaviour at the time.
It began as the Stockyard Creek fire on November 25, and merged with six other blazes throughout December, Ms Ward said.
Lightning data showed strikes near Stockyard Creek Road on November 22, but a fire was not discovered by aerial crews until three days later.
Paul Sandilands, who was the National Parks and Wildlife Service's air attack supervisor for that region, said it was not unusual for lightning-related fires to go unnoticed at first.
"When lightning hits the ground, or hits a tree, it doesn't immediately start a fire," Mr Sandilands told the inquiry at the coroner's court, sitting in Katoomba.
"A fire could sit for any length of time before it takes a run and expands. A tree could sit and smoulder for a couple of days until the fire grows."
He said it would be difficult for anyone to access the dense bushland where the fire started, and a campfire or smoking were unlikely causes.
Detective Senior Constable Katie Platt, the officer in charge of the investigation, said although police were unable to get to the ignition point, the fire was "100 per cent" caused by lightning.
The inquiry is due to move onto the cause of a fire in the Upper Turon, in the state's central west, on Tuesday.
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