Four people were rescued from the two-storey building on Commerce St, including two men who climbed onto the roof to escape the fire.
One young woman, who braved the smoke-filled corridor to wake her neighbours, was taken to hospital for smoke inhalation and a cut hand, sustained as she tried to smash glass to access the fire alarm.
Two firefighters were treated at the scene for cuts and exhaustion.
Fire investigator Mike McEnaney said the blaze was being treated as suspicious and had been referred to police.
Mr McEnaney said the fire exits were adequate but it appeared the alarm system was not working properly.
"That's a very serious concern, because the purpose of those is to give early warning to evacuate people," he said.
Building manager Jonathan Kim said the alarm was tested monthly and he believed it was working.
The blaze was reported to the Fire Service about 7.15am.
One resident, Jamie Whittaker, woke as smoke filled his room.
"I (saw) flames coming up through the roof, up through the skylight, so I woke everybody up and said, 'We've got to get out of here, there's a fire'.
"Everyone started getting up and panicking, running around trying to put clothes on, things like that. Two of them went up onto the roof and me and another one smashed the back window.
"We were going to jump down because we were one floor up and the walls were starting to get really hot but then fortunately the fire guys came round just in time and put the ladder up and we were able to climb down."
He said he feared for his life as he tried to escape the smoke and flames.
"We were very close, definitely. We couldn't get out the fire escape because the fire escape was blocked."
He did not know the extent of damage to his apartment or possessions.
Another resident, Simon Che de Boer said he went into "fight or flight" mode and climbed a ladder onto the roof.
He said he felt like a "gargoyle" as he clambered around the hot roof looking for a safe spot.
"My immediate concern was I was going to step on something, that I was going to fall ... because it was, like, you could feel it [the fire] beneath you."
He found a corner of the roof away from the heat and firefighters were able to get him down on a ladder.
His girlfriend did not go onto the roof with him and instead rushed down the smoke-filled corridor to wake other residents.
"She actually ran into the smoke - ran into the black - knocked on the neighbours' doors, knocked on the other neighbours' doors, in complete blackness.
"If it wasn't for her I don't think the neighbours would have made it out ... they would have been fast asleep, they wouldn't have known."
She was taken to hospital to be treated for smoke inhalation, but was "okay", Mr Che de Boer said.
"I don't think my flat probably made it. I've lived there for seven years, I have a lovely apartment - I had a lovely apartment. I don't know."
Vera Yang, her partner and their young son were sitting on the pavement outside the building with a laundry basket containing some clothes and a suitcase - all they had managed to grab as they fled.
"I was having a shower and I smelled something burning. I thought my son was using the toaster but he wasn't so I looked out the windows and there was so much smoke outside," she said.
"It was very frightening. I was just thinking what we could do, just run, grab clothes and laptop and just run."
She did not know whether their apartment had been damaged or destroyed.
"We're just waiting," she said.
