The hot weather comes as firefighters battling the Currowan fire at the South Coast were dealt a blow on Sunday with the fire escaping containment lines.

It forced the closure of the Kings Highway as a one kilometre fire front moved quickly uphill towards the road in the Misty Mountain Road and Government Bend area.

The fire had been contained by the Currowan Creek but it was an awkward spot for firefighters and gusty afternoon winds carried the fire across the creek.

It was a disappointing occurrence for firefighters ahead of what will likely be a difficult week due to the extreme heat.

The hottest December temperature previously recorded in the ACT was 39.2 degrees in 1994. We're expected to exceed that twice this week with 41 degrees predicted for Thursday along with Saturday's 43.

Bureau of Meteorology forecaster Helen Kirkup said a very hot air mass in central Australia was pushing east and would travel across NSW and the ACT throughout this week.

"There is a chance a lot of places will break [temperature] records," Ms Kirkup said.

It was too early to tell when the extremely high temperatures would abate, she said, as they extended beyond the end of the forecast period.

Cold fronts across the region were moving too far to the south, Ms Kirkup said, and it would require a cold air mass further north to lower temperatures.

"It's a bit of a wait and see, there's no firm ideas on [when it will end] yet," she said.

Canberrans should expect more smoke across the territory, particularly on Monday and Tuesday nights with easterly winds predicted.

Smoke has been forecast to return to the capital on Monday afternoon.

Monday's temperature is expected to reach 32, according to the Bureau of Meteorology with winds of up to 35km/h predicted later in the day.

The mercury will rise again to 34 on Tuesday and increase to 38 on Wednesday, before nudging into the 40s on Thursday.

The high temperatures will pose a risk to the two fires burning to the east of the ACT, the North Black Range fire near Braidwood and the Currowan fire at the South Coast.

Rural Fire Service Lake George District officer Darren Marks said the low humidity and wind this week, particularly on Thursday and Saturday presented a risk the North Black Range fire could flare up again.

Conditions over the past week had been, on the whole, far more amenable for firefighters to implement and strengthen containment lines, both near Braidwood and at the coast.

The North Black Range fire was labelled under control.

Mr Marks said the RFS was very conservative in making the decision to label the fire under control ahead of the severe conditions this week.

He said crews would continue working hard to ensure the fire did not pose a threat with the worsening conditions and would act immediately if any there were any new ignitions.

He said the RFS remained "confident but not complacent" that the containment lines would hold the fire despite the conditions.

"There's no guarantees in this business," Mr Marks said.