Two people were seriously burned on Friday night, in an explosion at Oak Flats that was heard a suburb away - in Albion Park.
Lake Illawarra established a crime scene after the explosion, which will be examined by specialists and detectives are investigating the circumstances surrounding the incident.
Six ambulance crews were called Forrest St, Oak Flats, where a man and a woman were suffering serious burns.
It is understood the man was walking towards a shipping container storing large pieces of wood, a car body and other items at the rear of the house when he was blown back by the explosion.
At this stage police believe to be caused by a leaking gas bottle that exploded when the fumes came into contact with a lit cigarette.
The bottle has been taken for analysis.
The 46-year-old man suffered burns to 50 per cent of his body and likely damage to his airways.
A 47-year-old woman also had burns to her body and airways and suffered an eye injury.
They were airlifted to Royal North Shore Hospital from Shellharbour Airport, where they remain in a critical but stable condition as of Sunday afternoon.
Fire and Rescue NSW Superintendent Chad Wallace said it was a "flash explosion", that lasted a few seconds and was not accompanied by fire.
He said neighbouring homes suffered superficial damage to tiles and windows.
People in nearby suburbs reported on social media that they heard and felt the explosion on Friday night.
"That shook our house and the neighbours and we are nearly a kilometre away," Simon Mills said on Facebook.