The bushfire smoke that has tormented Canberra since late last week returned on Thursday night and will hang over the city for most of Friday.
The Bureau of Meteorology says the smoke haze wasn't expected to clear for several days.
The haze was driven by an easterly wind, blowing smoke from the North Black Range fire near Braidwood and the Currowan fire on the South Coast over the ACT.
As of 6am on Friday, ACT Health's air quality rating at stations in Florey and Civic have been labelled as very poor.
Air quality is slightly better at Monash, which has been rated as poor.
Bureau of Meteorology forecaster Helen Kirkup said while some of the smoke would clear during Friday afternoon, it would return on Friday night.
"During Friday as the ground heats up and the smoke mixes out more vertically, the winds are expected to come from the north-west, and that should see a lot of the smoke dissipate," Ms Kirkup said.
"However, after sunset on Friday we expect the easterly winds to come in again, and there's the chance of more smoke tonight assuming the fires are still burning."
The smoke is expected to stick around over the weekend and into Monday.
"On Saturday there'll be a stronger westerly, but the fire won't be far away and the winds may come back in," Ms Kirkup said.
"The next couple of days until Monday has more of an easterly coming in, and the smoke is going to fluctuate, depending on the winds.
"There's still a decent source of smoke and the westerly winds will fan the flames but the easterlies will bring the smoke back."
It's expected Sunday will be the best day in terms of air quality.
Canberra spent the early part of the week blanketed in smoke from the fires, including from the Currowan fire burning on the South Coast.
Both the Currowan fire and the North Black Range fire near Braidwood are at advice level.
Earlier this week, the air quality was rated hazardous on ACT Health's air quality website, and the Education Directorate recommended all outdoor activities at schools be cancelled on Tuesday and Wednesday due to the smoke.
The ACT Health Directorate urged people who are more sensitive to the effects of smoke, particularly those with asthma and other heart or lung conditions, to take care.
"This includes taking medication as prescribed by their doctor and avoiding vigorous exercise outdoors during smoky conditions."
Canberrans have been advised to stay indoors with the windows and doors closed and stay in airconditioned buildings with the air conditioner switched to recycle or recirculate.
The Emergency Services Agency said there were no grass or bush fires burning in the ACT as of 8am on Friday.
A top of 26 degrees is forecast for Friday, with 31 forecast for Saturday and Sunday, and 33 on Monday.