Police and fire investigators have returned to the area today, which has been declared a crime scene.
The fire began at 4:30pm (AEST) on Sunday in the Cathedral Place unit complex in Gotha and Ann Street in Fortitude Valley.
The complex is made up of five blocks of units. Fire inspectors say the blaze started in the ceiling of one unit and spread to others on the top level of G and H blocks.
It took firefighters an hour to control the blaze and several more hours to extinguish.
Hundreds of residents were evacuated to a nearby park.
Fire investigator Simon Young says the fire was caused accidentally and started in an upstairs bedroom while a woman was in the unit.
She is in hospital recovering from smoke inhalation.
Mr Young says it could have been caused by a candle.
"That could be a possibility - I believe the person was perhaps in that room at the time," he said.
"It can be extremely difficult when there's that amount of damage in a fire, so we might not be 100 per cent able to determine the cause, but we're pretty comfortable it was an accidental fire."
Insurance assessors are on scene to determine the extent of damage.
Tenant Chris Walker escaped from the burning building and is worried about when he can return.
Caretaker Paul Ellis says the emergency response was brilliant.
"[Queensland fire service] and all the police services have performed very efficiently and they have all been 10 out of 10 in making sure everyone was safe, which is the most important thing," he said.
"Cathedral Place is one whole city block - 520 units, 1,200 people who live here - so it has been a pretty large-scale evacuation."
Resident Tyson Hoelscher has just been allowed to return to his apartment.
He says he watched the fire travel from the first unit to the plant room where it exploded.
"I've heard a lot of rumours and stuff but I looked in that unit from here and I could see no-one in there," he said.
"I say I smelt a lot of electrical smell - you know what I mean - and to me it may have been an electrical fire."
Authorities say the majority of residents have been allowed to return to their homes, but hundreds of people who live in the fire-damaged blocks have been told it could be weeks before they can go home.
Mr Ellis says accommodation is being provided for those who cannot return to their homes.
"The local PCYC at Brook Street has offered their assistance as an evacuation point and evacuation crisis centre that may need assistance with blankets and water or even accommodation," he said.
"People are stepping up to help the local community - I think that's great."
Boystown general manager Wendy Protheroe says arrangements have been made for people left homeless by the fire to stay at an evacuation centre if need be.
"Most of them are going to stay with friends around Brisbane," she said.
"But for those who aren't, we've either got people going down to the Fortitude Valley PCYC or in some cases they are staying in apartments in the city."
Ms Protheroe says those residents are being supported.
"Obviously we have got a bit of tension coming in now - there are people that quite stressed, but we are doing our best," she said.
"Police have been marvellous, the fire brigade have been terrific.
"We have a chaplain here and we are just organising people the best way we can, so we know they are getting as much support as they can."