Firefighters had been investigating reports of smoke in the Brindabellas south-west of Canberra.

The smoke was sighted near the ACT/NSW border in the evening.

A spokeswoman for ACT Emergency Services Agency said there had been storm activity in the region.

"Following several lightning strikes, firefighters manning two of Canberra's fire towers, Mt Tennant and Mt Coree, reported possible smoke near the border in New South Wales," the spokeswoman said.

"A helicopter flight over the area did not detect any fire."

A further check on Sunday also found no fire.

"A helicopter flight over the area this morning [Sunday] did not detect any fire or smoke following lightning strikes near the border last night. The ACT Rural Fire Service is remaining vigilant to the possibility of fires from the evening lightning strikes and is conducting an extensive aerial sweep along the path of the storm," the agency said in a statement.

ACT Rural Fire Service trucks are on standby at Mount Franklin Road as a precaution.

Saturday and Sunday are both declared total fire ban days.

On Saturday afternoon, the NSW Rural Fire Service said 49 bushfires or grassfires were already burning across NSW, 17 of which were not contained but no loss of life or property had been recorded.

NSW faced the "worst possible fire conditions" in its history with a "catastrophic" warning in place across large slabs of the state after a heatwave smashed temperature records on Saturday.

Rural Fire Service (RFS) Commissioner Shane Fitzsimmons said the situation was as "bad as it gets" and warned it was set to get worse on Sunday when winds are expected to sweep through scorched parts of mid to northern NSW.

"To put it simply [the conditions] are off the old scale," he said. "It is without precedent in NSW".

For storm assistance call 132 500.

More to come.