The terrifying bushfire emergency that hit the Blue Mountains late last night happened after the RFS lost control of a "absolutely crucial" backburn, the volunteer fire service admitted.

At least 20 buildings are believed to have been lost last night as the megafire jumped containment lines.

Firefighters had been conducting hazard reduction burns along the southern edge of the Gospers Mountain fire – which has burnt close to 400,000 hectares of bush – when the flames spread.

“There was backburning in the area and unfortunately due to the conditions they’re burning in, it’s taken one ember to cross over the wrong side and that’s where it took a run and it was escalated to emergency warning and then we have had reports of homes lost in the area,” an RFS spokeswoman told AAP.

Reports have also surfaced that some firefighters out trying to protect their local communities instead lost their own homes to the megafire.

A firefighter continues to cool and protect the surroundings of the industrial shed destroyed this afternoon. Picture: Matrix

Speaking to The Sydney Morning Herald, RFS spokeswoman Angela Burford described the risk of ember attacks on homes and spot fires as "the risk we take".

Embers jumped over containment lines on Sunday afternoon with spot fires quickly spreading to homes. An unknown number of properties were destroyed from Bilpin to Mount Tomah.

Mount Tomah Botanical Gardens also suffered damage from the fire.

The Gospers Mountain fire front could be seen from the botanical gardens. Picture: Matrix

RFS Commissioner Shane Fitzsimmons said it's possible "dozens" of buildings had been lost and described it as an "awful scenario".

“We have reports of dozens of building, structures having been damaged or destroyed,” he told The Today Show.

“I am hearing reports that even our local members and it might be our local captain have lost their home last night trying to defend their local community so that's another awful scenario being played out."

A destroyed shed in the Blue Mountains region. Picture: Matrix

Commissioner Fitzsimmons later spoke to the ABC about the efforts to stop the fire crossing the Bells Line of Road but "unfortunately things don't always go to plan".

As of 12pm today, the fire was still active on both sides of the Bells Line of Road and is now burning in the Grose Valley.

The threat was downgraded as not having an immediate threat by Monday morning.

But authorities have warned there’s a risk that embers could fall in the Bilpin area, starting new blazes ahead of the main fire front.

“I’ve not seen fire to this scale… it’s unprecedented to see. We’re just shy of 3 million hectares… that’s burnt so far this season.” RFS Commissioner Shane Fitzsimons on fears that the homes of some firefighters were lost while they bravely defended their communities. #9Today pic.twitter.com/THj6hlE8DK — The Today Show (@TheTodayShow) December 15, 2019 https://platform.twitter.com/widgets.js



