'Our worst day': Dangerous fire threatens homes as heatwave hits

Authorities fear a dangerous blaze could jump containment lines and destroy homes as the ACT prepares to battle its worst bushfire conditions in more than a decade.

The bushfire conditions forecast for Saturday are being compared to those experienced in 2003, when four people died and hundreds of Canberra homes were razed.

The Orroral Valley fire southeast of Canberra had burnt through more than 28,000 hectares as of 2am Saturday morning, with a state of emergency declared for the ACT.

Rural properties in southern parts of the territory came under threat from the fire late on Friday night, but an emergency warning was later downgraded.

ACT Emergency Services Commissioner Georgina Whelan says there is the potential for the fire to spread on Saturday.

"This does include potentially as far north as Tharwa and spotting close to the southern suburbs of Canberra, including Banks," she said on Friday night.

A "textbook" combination of dangerous fire conditions are forecast for Saturday, Ms Whelan said.

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"It has been unpredictable. It remains challenging. It is difficult to access. This fire could create its own weather system. The predictions are that Saturday will be our worst day.

"If all of what I have just outlined occurs, there is a chance this fire could break containment lines."

Saturday ‘greatest concern’

Chief Minister Andrew Barr declared the state of emergency with the fire forecast to spread north towards urban areas over the weekend.

"Today is the day of greatest concern," Mr Barr told the Nine Network on Saturday.

"But it is forecast to be still quite hot tomorrow (Sunday) and we are expecting some storms that could lead to quite unpredictable weather, wind changes."

Mr Barr said while the state of emergency is expected to remain until at least Monday, there has been a "tremendous preparing effort" to protect both heritage and properties in the area.

But he warned the "the winds are up, so the fire is awake”.

He said the winds are coming from the northwest so the fire is going to track towards NSW.

"This is a really big fire and it is expected to move across the ACT border into New South Wales," he said.

Ten per cent already burned

At the same time, the fire is burning slowly to the north against the wind and approaching Tharwa with spotting ahead of the fire.

Over 10 per cent of the territory has burned out by the bushfire.

The weather forecast for the ACT is for 40 degree-plus heat and gusty winds on Saturday, which the Bureau of Meteorology's Dean Sgarbossa says along with gusty winds and dry conditions "will drive fire dangers into the severe area".

However, he told ABC television humidity will increase throughout the day which will cap fire dangers heading into Sunday.

Australia’s east coast sweltered through Friday with temperatures forecasted to climb into the 40s across NSW, Victoria and the ACT throughout the weekend.

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