Firefighters battling blazes in regional New South Wales have stumbled across a hidden field of massive marijuana plants.

The NSW Rural Fire Service was near the town of Tahmoor, west of Woollongong, when fire crews made the discovery on Friday.

The firies took selfies with the naturally occurring plants and posted them online.

The photo suggested the firefighters were amused - the surprising find being a welcome distraction from the fires plaguing the area.

Firefighters battling blazes in regional New South Wales have stumbled across a hidden field of massive marijuana plants

The caption suggesting firefighters were amused - the surprising find being a welcome distraction from the fires plaguing the area

The find was made near where two firefighters tragically lost their lives when the fire truck they were in crashed.

Volunteers Andrew O'Dwyer, 36, and Geoffrey Keaton, 32, were killed on Thursday evening when their truck rolled off the road after hitting a fallen tree at Buxton, south of Sydney.

Their colleagues were seen sharing embraces and wiping away tears at Horsley Park Rural Fire Brigade in Sydney on Friday.

Mr O'Dwyer and Mr Keaton, who were both fathers, had been fighting fires for more than a decade and were remembered as heroes by Prime Minister Scott Morrison, who is in Hawaii on holiday.

'They were bravely defending their communities with an unmatched spirit and a dedication that will forever set them apart amongst our most courageous Australians,' Mr Morrison said in a statement.

'Their sacrifice and service saving lives and saving properties will be forever remembered. I wish those injured all the best in their recovery.'

It's feared some 40 homes were lost in Buxton, Balmoral, Bargo and surrounding areas on Thursday, as the Green Wattle Creek blaze tore through the Wollondilly Shire.

That blaze remained at emergency warning level early on Friday, along with the huge Gospers Mountain fire in the north-west of Sydney.

The neighbouring Kerry Ridge blaze and a fire on the NSW South Coast at Currowan were downgraded to a 'watch and act' alert.

Conditions worsened on Friday which was expected to eclipse the hottest day on record for a third consecutive day.

A searing heatwave combined with 100km/h winds fuel one of the nation's worst ever bushfire crisis.

Fire crews are bravely battling to slow the spread of the blazes.