Three young children have died overnight after a house fire at Lalor Park in Sydney's west in what is being treated as a domestic violence incident.

Fire crews were called to the scene just before 1am.

Police allege a 28-year-old man, who is now in custody, tried to stop police and other emergency services from rescuing the children from the burning home.

"I can confirm during police attempts to get into the property, those efforts were frustrated by a male inside," NSW Police Acting Superintendent Jason Pietruszka said.

The man arrested is the father of the children who died and remains under police guard being treated for smoke inhalation in hospital.

Police said two boys, aged two and four, were treated at the scene by paramedics and taken to Westmead Hospital in a critical condition, but died a short time later.

Fire and Rescue crews extinguished the fire before the third child, believed to be a 10-month-old girl, was found dead at the scene.

Police say the children have not been formally identified.

Four other children, all aged six to 11, and a 29-year-old woman, have been taken to hospital.

NSW Fire and Rescue superintendent Adam Dewberry said firefighters rushed into the burning home to search for the children.

"When our crews arrived they were confronted by pretty chaotic scenes with a number of adults and children who had self-evacuated," he said.

"There was heavy fire activity, the flames venting out through both front windows, getting up over the roof line.

"They got in under those fire conditions, started to knock down the fire and conducted a search under very extreme conditions."

The firefighters who found the two and four-year-olds went back into the home to search for the baby.

Superintendent Dewberry said temperatures likely reached between 600 and 700 degrees.

"They can't see … they're getting through under extreme, superheated conditions, while looking for those children," Superintendent Dewberry said.

"The firefighters who found the two children actually went back in to conducted a search for the third child, but unfortunately they were unable to locate that child, who was located after the smoke had cleared."

He described the scene as "confronting" for emergency services.

"I watched a young child sitting on a young constable's lap while they were being treated by paramedics," Superintendent Dewberry said.

"This is just a tragedy all round. Everyone deals with these things differently; we've got to make sure that we are there for our firefighters, there for each other."

Shocked locals woke to the scene this morning.

Nearby residents Graham and Elizabeth Cocks said they are devastated by the deaths.

"It's just terrible," Cocks said.

"When it happens so close to home, it's happening in the suburb we have been for 20 years, just devastating to know that this sort of thing has gone on … just awful."

- ABC