Up to 90 firefighters, a large number of police and paramedics raced to the scene within minutes of the automatic fire alarm going off at 5.55am (AEDT) (7am NZT).
Staff were already evacuating residents when firefighters arrived to find a wing of the home burning "fiercely".
The cause of the fire is under investigation.
"This is a firefighter's worst nightmare," he said, adding that many of the elderly people could not get "out of harms way".
The commissioner said the firefighters had to fight their way through thick black smoke, and were on their hands and knees trawling through rooms for people trapped inside.
"(They) could not see their hands in front of their faces."
He said when crews arrived at the scene they found a "serious fire in progress". He described how the building was full of smoke, with fire fighters unable to see and having to search the home on their hands and feet.
Many of the residents have been hospitalised for burns and smoke inhalation.
Dozens of elderly people are lying on beds or sitting in wheelchairs outside the home, some of them being attended to by paramedics.
The driveway to the home has been turned into a makeshift triage centre.
Staff and what appears to be many concerned relatives are also waiting outside.
Dozens of ambulances and fire trucks have attended the emergency and remain on scene.
Mr Mullins said it would have been a horrific experience for the elderly people, many of whom are in wheelchairs and beds, huddled outside the burnt out building.
"They are confused... they are not sure what is going on," he said.
"It is just horrific, absolutely horrific," one neighbour told AAP.
"Tremendously sad that this has happened to gentle elderly folk."
Comm Mullins said the cause of the fire was under investigation.
The state coroner and arson and homicide investigators will attend the scene.
