ACT Fire and Rescue senior commander Brian Talbot said a crew isolated a fire on Sunday caused by a nine-kilogram gas cylinder on the balcony of an apartment on Beissel Street, Belconnen.

"Initial investigations showed that the gas hose had likely perished, causing a gas leak," he said.

"It's a reminder to check that gas hoses and equipment haven't perished in the winter months."

Firefighters also attended an incident in an apartment on New South Wales Crescent at Forrest on Sunday night, after a small amount of paper left on a cooktop that was accidentally left on caused a slow-burning, smouldering fire.

"We were able to hand the apartment back to the occupants with minimal damage," Mr Talbot said.

In a separate incident on Sunday night, a small grass fire was extinguished at the Mt Taylor Nature Reserve.

The cause of the fire is unknown and investigations were continuing on Monday morning.

The fire was extinguished shortly after fire crews were called.

A low to moderate fire danger rating on Monday will give way to a high fire danger rating for Christmas Day, Boxing Day and December 27.

Temperatures will climb to 38 degrees on Friday, after a high of 33 degrees on Tuesday, 35 on Wednesday and 37 on Thursday.