As of 6.30am on Sunday, the Orroral Valley fire remains at a watch and act level after conditions eased overnight on the fireground.

However, emergency authorities are warning conditions could get worse later in the day.

The fire is 55,234 hectares, almost 23 per cent of the ACT, and continues to travel in a south-easterly direction.

The ACT Emergency Services Agency has warned properties may be impacted by embers well ahead of the main fire.

Meanwhile, the Clear Range fire burning in NSW remains at an advice level and conditions have eased.

While the immediate threat to properties and lives in Michelago, Bredbo and Colinto have eased, the NSW Rural Fire Service is warning for residents to be vigilant.

Firefighters are continuing to slow the spread of the fires.

ACT Emergency Services Agency commissioner Georgeina Whelan said while overnight conditions were favourable to fire crews, difficult conditions were ahead.

"We're not out of the woods yet," Ms Whelan told ABC radio on Sunday morning.

"We've been teasing the fire out of Mount Tennent and with the five containment lines between the foothills and Tharwa have allowed us to directly attack the fire."

Fire crews have been working throughout the night to edge the fire out over Mount Tennent to allow firefighters to directly attack the fire head on.

However, Ms Whelan warned of further erratic fire behaviour, ahead of a storm expected to pass through the ACT on Sunday.

"We have challenging weather ahead in the next week conducive to active fire activity," Ms Whelan said.

The fire remains 1.2 kilometres from Tharwa and six kilometres from southern suburbs of Tuggeranong

The biggest threat for residents in the ACT was the Orroral Valley fire, which was upgraded to emergency alert at 3pm in the afternoon on Saturday. Temperature soared to 42 degrees and ACT Emergency Services commissioner Georgeina Whelan warned of increasing fire activity, spotting and erratic behaviour on all fronts of the massive blaze.

But by 4.45pm, the fire was downgraded to watch and act and Ms Whelan was buoyed by what she described as "a very good firefighting day for us".

Fire activity was moderate during the evening. Rain is forecast for Sunday, with the possibility of storms. The temperature is set to drop slightly. The humidity increase and the rain was good news, Ms Whelan said. But strong winds were also forecast.

"I'm really liking the sound of the rain, but we're going to have to be very focused on the winds," she said.

The fire was burning about two kilometres from Tharwa and seven kilometres from Banks late on Saturday night.

The picture was not as good for areas near Bredbo, south of Canberra on the Monaro Highway, where fire ripped through rural properties.

A 2.5-kilometre barrier of retardant was dropped around the base of Mount Tennent to protect Tharwa on Saturday, laid in three drops by a DC10 operating from Canberra Airport. Ms Whelan said the defensive perimeter would slow down the fire.

On Sunday a retardant line would be laid between Tharwa and Gordon, in southern Tuggeranong.

Flames were visible to residents on Saturday evening but the flames were from backburning operations being conducted, designed to slow the fire's progression towards Tharwa.

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During the day on Saturday, firefighters were restricted to aerial firefighting while the fire burned on Mount Tennent. But they planned to attack it on the ground when it reached the grasslands. At 8pm, it was less than 200 metres from the grasslands and expected to reach the area on Saturday night. At that point, firefighters would be able to "take some very decisive action" with active suppression, Ms Whelan said.

All structures had been protected in the ACT, including historic homesteads and alpine huts. Earlier, the fire was burning over the road from Namadgi Visitors Centre and spotting into the mulch beds, but the centre was saved. Critical infrastructure was also intact, with a strike team sent to Williamsdale to protect a solar farm there.

People in Apollo Road, Boboyan Road, Naas Road and Top Naas Road should remain vigilant. People in Tharwa should monitor conditions, and people in Banks, Conder and Gordon should stay up-to-date.

But Canberrans could also "sleep well tonight [Saturday] knowing there are 2000 people out there to keep you safe", Ms Whelan said.

The fire reached 55,200 hectares, about 23 per cent of the ACT's land mass. It had grown almost 20,000 hectares in just five hours, was out of the control and was travelling in a south-easterly direction.

Sixteen aircraft were fighting the fire, including DC10 and 737 large air tankers, helicopters and water bombers.

The ACT installed a large air tanker base at the Canberra Airport 18 months ago, allowing the aircraft to refill with retardant at Canberra Airport instead of having to make the longer flight to Sydney, Emergency Services minister Mick Gentleman said.

An emergency alert remained in place on Saturday evening for the fires near Michelago and Bredbo. The NSW Rural Fire Service said the fires had started as spot fires from the Orroral Valley fire in Namadgi and covered more than 5000 hectares.

The fires were moving quickly in a southeasterly direction towards Bredbo, with an immediate threat to properties and lives in the areas of Colinton, Michelago and Bredbo, and the Monaro Highway closed. The fire crossed the highway near Colinton.

Homes and outbuildings were lost in the area between Bredbo and Michelago, NSW Rural Fire Service community liaison officer Beth Slender said.

"We know that with the fire behaviour that we've seen and the area that's been impacted, that it is highly likely that we will see some structural losses," she said.

The numbers were unclear.

In the Colinton and Bumbalong areas north of Bredbo, people on rural properties spoke of the ferocious appearance of fire on Saturday. While they were prepared for fire, the suddenness and speed with which it arrived took them by surprise.

A little further south, at the Macenmist truffle farm and vineyard, about 10 kilometres east of Bredbo village, Barbara Hill was readying for a long night, with the Clear Range fire about 8km away.

In the early evening, she said fire-blackened leaves were falling on the property, and the wind was unpredictable, swirling in different directions.

The family had fires on both sides, with the Good Good fire still burning on the coast side, and the possibility the fire fronts would join in the Jerangle area, creating a vast swathe of fire from Namadgi to the coast.

"The sky is black, there's lot of smoke and the wind is still blowing around," she said.

They were prepared, with sprinklers on the roof, the area around the house cleared and the three truffle dogs inside. But they were in for a long night.

Mrs Hill said while authorities were issuing frequent warnings, maps were not being updated quickly enough to show where fire was burning. The fire had already hit three properties of people she knew.

Canberra hit a top temperature of 42 degrees at 2.22pm and wind gusts reached 61km/hr.

Bureau of Meteorology forecaster Neil Bennett said the forecast thunderstorm, and the possibility of severe conditions related to that, could cause problems on Sunday.

"We think the worst of the fire conditions are today, but we're not really expecting to see cooler conditions moving in with a drop in the fire danger risks until Monday," he said.

"The thunderstorm risk tomorrow adds to the complexity."

The thunderstorm could potentially bring some localised heavy rainfall.

"There will be very difficult firefighting conditions again tomorrow," Mr Bennett said. "There is no room for complacency."

Saturday night's minimum temperature was forecast to be 25 degrees, with the temperature to reach 34 degrees on Sunday and 30 degrees on Monday.