He remained in hospital on Thursday in a critical but stable condition.

Detective Senior Sergeant Dianne Dale told reporters at the scene that investigators will "determine the point of origin, and what led to the circumstances being suspicious".

An Ambulance Victoria spokeswoman earlier confirmed an adult woman and a primary school-aged boy were assessed at the scene for smoke inhalation and taken to the Austin Hospital.

"They were both in a stable condition," she said.

An MFB spokeswoman said that the fire brigade "did a little bit of venting of smoke ... the call was deemed suspicious so police are investigating".

Next-door neighbour Frank said he did not hear any loud noises at the house overnight but woke to see the flashing lights of the fire trucks outside his home.

"I saw an ambulance, a couple of trucks so I went out and I asked them what happened and he said there was fire in the bedroom of this house. I asked how were the people, the woman's OK, boy is OK, man is a bit burnt," he said.

Frank said he doesn't know the neighbours well, but said they were friendly.

"I say 'hello', I had a chance to talk to them a couple of months ago about the new fence. I went to the house and talked to them. The wife told me she got cancer, she's sick. The husband is a nice man," he said.

Other neighbours told reporters that family had moved to the area from China a few years ago and that the woman had recently been diagnosed with cancer.