Four children under 10 have died after becoming trapped inside their home as it was engulfed in flames in southwest Melbourne.
Firefighters were called to Mantello Dr in Werribee just before 1am on Sunday, to find the roof of a single-storey brick house on fire.
Werribee fire brigade Lieutenant Damien Molloy said when crews arrived the property was "very well alight".
"Fire had already broken through the roof and was already coming out the front door," he told reporters on Sunday.
Initial police reports said four children were unaccounted for.
Lt Molloy said crews were aware the children were trapped inside the house, but the intensity of the blaze made it difficult to find them.
"With the roof already being compromised and the tiles collapsing, it was unsafe for our members to be able to proceed initially," he said.
"With the distress of the parents and eight-year-old, it was difficult to get accurate information early on to focus our search areas."
Two girls, aged one and six, and two boys, aged three and 10, were found dead inside the house.
Their parents and brother managed to escape the fire and are in hospital.
The man, 45, suffered serious smoke-related injuries, the boy, 8, minor injuries and the woman, 38, suffered smoke inhalation.
One firefighter was assessed by paramedics for chest pain.
It took more than 30 firefighters just under an hour to bring the fire under control, with the house completely destroyed.
Lt Molloy said fire crews felt "in a way, a little bit helpless" because they wanted to get into the house to find the children.
"The fire itself we have seen many times before, but the complexity involves having four children trapped inside the house," he said.
"We're not wanting to put ourselves at that level of risk to have, what was likely to be, a recovery situation from the outset."
He said, while there were "a lot of people in the street" at the time, neighbours could not have helped extinguish the blaze due to its intensity.
Victoria Police arson and explosives detectives are investigating the cause of the fire and will prepare a report for the coroner.
Detective Senior Sergeant Ashley Ryan said he could not yet say whether the fire was suspicious.
He said investigators would speak to the surviving family members at hospital.
"It is a terrible set of circumstances and one that is very difficult for members of the family, and our hearts go out to the family and to the community this morning," Det Sen Sgt Ryan told reporters.
"But also very distressing for members of Victoria Police and emergency services, given the circumstances of what's occurred."
Reports that it may have taken 10 minutes to get through to triple zero after the blaze erupted are understood to be false.
AAP understands the call was answered within 79 seconds.
An Emergency Services Telecommunications Authority spokesman said triple zero operators are "always distressed if any callers have to wait for help".
"This is amplified if our most vulnerable people, like children, are involved," he said.