Firefighters risked their lives when they heard three people were inside a building that was engulfed by a fire which is suspected to have been deliberately set.

Papatoetoe senior station officer Terry Jenner said he feared the worst after being told there was a family inside when he arrived at the small three bedroom cottage on Redcrest Ave, Papakura, about 11.30am yesterday.

"The fire was very involved when we got there, I was pretty sure no one would have been able to survive it."

Nonetheless, firefighters entered the building in the hope someone had somehow survived the inferno, but found the rooms empty.

One firefighter fell through the floorboards, which had been weakened by the flames, and the roof looked like it could collapse at any minute, Mr Jenner said.

But the family, including two young children, arrived home while the blaze was being extinguished, much to the firefighters' relief.

Fire risk management officer Phil Faidley said about 90 per cent of the house's interior was destroyed by the blaze and it would probably have to be demolished.

The fire was being treated as suspicious due to "distinctive patterns of burning that point to a deliberately set fire" and had been referred to the police.

There were no smoke alarms in the house, he said.

"If there had been people in the house there is a high likelihood there would have been a fatality."

Immediately after extinguishing that fire, the same crew was called to a blaze on Druces Rd, Wiri, where a mother sleeping in a bedroom with two-year-old twins was woken by a smoke alarm and found a "ball of fire" in the living room.

She managed to get outside with the babies but initially feared her four-year-old son could still be inside, but he was quickly found hiding in a garden shed.

The firefighters -- who were covered in soot and ash from their previous job -- managed to save the house's sleeping quarters but the living area was largely destroyed.

Mr Faidley said it appeared the young boy had started the blaze by playing with a lighter but investigators had not ruled out other possible causes.

Mr Faidley told NZPA he had attended five fires thought to have been started by four-year-olds since December 23.