Neighbours to a Gymea home that caught fire in the early hours of Saturday morning thought the residents were quarrelling when they were woken by yelling next door.
Then they saw a man running from the granny flat ablaze.
"He came running out of the building on fire and they were all trying to put him out," a resident of the next door units on San Remo Avenue said.
"It was really horrific."
The 39-year-old man's father, who lived in the main house, attempted to save his son, but by the time ambulances arrived shortly before 2am he had suffered burns to about 70 per cent of his body.
He was taken to Royal North Shore Hospital in a critical condition and police are investigating the cause of the fire.
It is understood that he was living in the granny flat with his girlfriend, whose cries of "help help" were initially misread as words raised in anger.
A next door neighbour, who asked not to be identified, said the fire was ferocious and quickly escalated after the first crackles were heard.
"I could hear a crackle and the smell of smoke and by the time I got out of bed and walked to my front door the whole building was - I can't describe it to you - just burning," the neighbour said.
"I just couldn't believe how quick it was.
"It was right up against our unit and the flames were double the size of our units. And of course then it exploded because it was fibro."
Neighbours spilled out onto the streets, holding onto their cats and dogs.
By this time the heat was so intense that the blinds in the lounge room of one neighbouring unit started to melt.
Firefighters advised residents to move their cars.
A spokesman for Royal North Shore Hospital said the man was no longer critical, but remained in a serious condition.
A police spokeswoman confirmed on Saturday that they were treating the fire as suspicious and there were a number of people they wished to interview, including the injured man.