The girl's 22-year-old mother was in a serious but stable condition at The Canberra Hospital on Monday night and her three-year-old brother in a stable condition.

Firefighters were called to the fire shortly after 2.30pm and found the girl's body inside at around 4pm.

Emergency Services said the mother and son were rescued from the balcony and "treated for minor burns and smoke inhalation".

But at 7pm, an ACT Health Spokesperson said their conditions were more serious.

Three police officers were also treated for smoke inhalation.

The victim's next door neighbour, Kenny Cameron, tried to bash down the door but was blocked by smoke.

"It just overwhelmed the place, it was so fast and rapid," he said.

"I feel so sorry for the young girl."

Inspector Luke Unsworth from Fire & Rescue NSW said when firefighters entered the unit, they battled "an intense fire with a lot flame" to search for the little girl until their gas cylinders ran out of air.

He said due to the type of material burning, much of it would catch alight straight after being extinguished.

About the time they ran out of air, ACT Fire & Rescue arrived to continue the search and found the girl dead.

"It is with great sadness today that we are here in Queanbeyan," Inspector Unsworth said.

"I've just visited the unit and had a look through, and it is immediately obvious to anyone who goes in there the intensity of the fire."

"It's lucky that we didn't actually have three fatalities from this fire."

Inspector Unsworth said residents helped firefighters find the best way to access the blaze, while NSW Police helped to quickly take the ladder off the truck to rescue the mother and son.

Firefighters did not find a working fire alarm, but said it was too early to tell whether one was installed.

They did not yet know the cause of the fire.

A woman who lives across the road, Elizabeth Bildin, said she heard screaming inside the unit at around 2pm and smelled a lot of smoke.

"I was terrified," she said.

"It is so upsetting to know that this has happened in our street. Nothing bad ever happens here."

Another neighbour, Abdulla Suman, said smoke was hanging high in the air from both sides of the unit and there was a group of people outside looking distraught.

"I could also see some peoples' faces in the windows of the other units and they looked upset," he said.

"It is very sad. I think to myself - what if this was my child?" he asked, while pointing to his toddler beside him.

Police are investigating and a report will be prepared for the Coroner.

- With Amanda Copp