Up to 20 buildings are feared damaged or destroyed in a huge bushfire north-west of Sydney that the NSW Rural Fire Service says started when it lost control of a backburning operation.

The homes of firefighters are thought to be among buildings destroyed when a controlled operation aimed at preventing the spread of the Gospers Mountain “mega blaze” breached containment lines on Sunday.

The affected properties are thought to be in the Mount Wilson and Bilpin areas.

“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 said on Monday.

RFS Commissioner Shane Fitzsimmons said earlier said that of the affected properties might belong to local service members. Assessment teams were expected to determine the exact damage on Monday.

The sprawling Gospers Mountain fire – which has burned 380,000 hectares – stretches from the Lithgow area in the west, through the Hawkesbury area and towards the Central Coast in the east.

It was at “watch and act” alert level on Monday afternoon having been downgraded from emergency on Sunday.

The RFS spokeswoman said the backburning losses were “absolutely unfortunate” but she added such operations were critical to help contain the vast fire before hotter conditions arrive later this week.

“If we don’t [backburn] then there’s nothing to stop this fire, and there’s heavily residential areas that it will look towards moving towards over the next days and weeks,” she said.

Endeavour Energy on Monday said 450 customers were without power in the upper Blue Mountains after the Gospers Mountain bushfire damaged powerlines.

Affected townships include Bilpin, Berambing, Mount Wilson, Mount Tomah and Mount Irvine.

Mr Fitzsimmons described it as an “awful scenario”.

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

“We are just not sure on homes and outbuildings and sheds and those sorts of things. 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.”

A heatwave forecast for this week would make for an “extraordinary amount of work still to go”, he said.

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Watch and Act: Gospers Mountain
The fire is active on both sides of the Bells Line of Road & is now burning in the Grose Valley. If you're in the areas of Mt Lagoon, Bilpin, Mt Irvine, Mt Wilson, Berambing & Mt Tomah, monitor conditions. https://t.co/ekOSH2RQ4v #nswrfs #nswfires pic.twitter.com/tq3bj96scv

— NSW RFS (@NSWRFS) December 16, 2019

Fires have already consumed almost three million hectares of land across NSW this season, supported by hot, dry and windy conditions.

Some 724 homes, 49 facilities and 1582 outbuildings have been destroyed. Six people have died and 2.7 million hectares have been scorched.

“The drought is having an absolutely profound effect on the vegetation, and its flammability. We are seeing fire burn effectively unimpeded under windy conditions,” Mr Fitzsimmons said.

The Bureau of Meteorology has forecast severe heatwave conditions for most of NSW throughout the week.

With more than 100 fires burning in NSW the RFS said it was particularly concerned about conditions from Wednesday onwards.

“There’s going to be a lot of work going on over the next 24 hours to try and just see what we can do to protect as many properties as we can, but people should be under no illusions,” RFS deputy commissioner Rob Rogers said on Sunday night.

“We’re going to have to get through this next week of really hot weather with these fires being very active.”

Firefighters protect a home north-west of Sydney from the Gospers Mountain fire a month ago. Photo: AAP

Ahead of soaring temperatures, Western Sydney Local Health District on Monday warned residents to keep well hydrated and cool to avoid heat-related illnesses and check in on elderly friends, neighbours and relatives.

“Extreme heat conditions can have serious impacts on peoples’ health,” WSLHD Centre for Population Health deputy director Christine Newman said.

As well as protecting homes, the RFS has been trying to save the Blue Mountains Botanic Gardens in Mount Tomah.

Most of the 28-hectare garden is safe but there has been some damage to plant collections and infrastructure.

Blue Mountains Mayor Mark Greenhill urged those in the affected areas to check on anyone living alone or who is less mobile.

“It is my fear, given the fires around our community, that we have very difficult weeks ahead. I am sorry to say that Christmas 2019 may not be free of the tension we all know too well,” he said.

-with AAP