Bungendore RFS Captain, Sheldon Williams.

Bungendore Rural Fire Service captain Sheldon Williams was back on the ground at the Sand Hills fire site on Thursday. "The heat is one thing, but it's the wind that can be the killer,'' he said. Photo: Rohan Thomson

Speaking over the low rumble of the fire tanker, Sheldon Williams gave an ominous forecast for the scorching days ahead.

"The heat is one thing, but it's the wind that can be the killer," he said.

"Friday will be our day."

The NSW Rural Fire Service Bungendore brigade captain is among the hundreds of firefighters preparing for worsening conditions across the region as forecasters predict a return to extreme temperatures on Friday.

A total fire ban has been declared for the ACT and most of NSW in the face of severe to extreme fire danger conditions.

For Mr Williams, an RFS member for 20 years, there were concerns that poor weather could prompt a repeat of the fires that tore through land near Bungendore last week.

"It wasn't the worst I've ever seen, but it was one of the fastest," he said.

"It moved very quickly from where it started to Hazeldell Road in an hour."

The Sand Hills fire off the Kings Highway has burnt through almost 1400 hectares, the largest blaze Mr Williams can remember in the area in about 15 years.

Exhaustion was also a concern as the fires wore on, he said. He had to balance tired members across at least 20 trucks at one time.

"[Fatigue] is an issue for any fire, particularly in the first day or two because people have come from their day jobs and started to pitch in," he said.

"You've got to be really careful to manage it, trying to rotate crews as quickly as we can."

A week on, most members have returned to their day jobs as crews complete some final mopping up in preparation for the worsening conditions.

"We're concerned and probably will be for the next two months," Mr Williams said.

"It's pretty quiet on the ground

now so, with luck, it's pretty much dead and hopefully nothing will stir up on Friday.''

In Canberra, crews are expecting severe fire danger as the capital climbs towards a predicted top of 39 degrees with winds of up to 40km/h.

The total fire ban, declared by ACT Emergency Services Agency Commissioner Dominic Lane, is designed to avoid burns that could become uncontrollable in severe conditions. An alert said such flames may be higher than roof tops. Severe to extreme fire danger is also expected throughout the regions, as firefighters tackling blazes near Yass and Cooma face down predicted highs of 39 and 37 degrees respectively.

The Cobbler Road fire burning 11 kilometres west of Yass has been classified as under control after moving through almost 14,000 hectares.

Crews are still controlling the Yarrabin blaze, which has burnt through more than 12,000 hectares since igniting almost a fortnight ago, but there are concerns it could jump containment lines if fuelled by expected gusts of up to 60km/h. The extreme heat and strong winds could be particularly challenging for crews, the Bureau of Meteorology's assistant director of weather services, Alasdair Hainsworth, said.

''The late onset of the monsoon has allowed extremely hot air to build up in northern Australia,'' he said.

''Fortunately, this weekend we're going to finally see some cloud cover and rain getting into northern Australia, and this will eventually flush out the hot air mass that has built up in the interior of the continent.''