“Once we pulled everyone out, we set up an exclusion zone. Our gas detectors went in there, and we started to identify carbon monoxide gas,” Fire and Rescue Superintendent Adam Dewberry said.

“We got a scientific officer there, and they’ve done a battery of tests, they’ve collected samples, and they’ve isolated an area that they’re interested in around a coolroom area … [The product has] been removed in that space.”

Haveli Restaurant owner Resham Singh speaking to media after a contracted cleaner died from carbon monoxide poisoning on Tuesday, September 16.

Haveli Restaurant owner Resham Singh speaking to media after a contracted cleaner died from carbon monoxide poisoning on Tuesday, September 16. Credit: Nine News

Restaurant owner Resham Singh said a cleaning contractor found the man lying on the floor, and called triple zero.

Singh said when he first arrived at the restaurant, opened just a year and a half ago, he was “shocked”.

“It’s so bad. I’m feeling so sad as well,” he said. “I’m still thinking, what’s going on?”

Carbon monoxide is a poisonous clear gas that can’t be tasted or smelled and can cause serious illness or death if inhaled at high levels.

Police start investigations outside the Haveli Indian restaurant in Riverstone.

Police start investigations outside the Haveli Indian restaurant in Riverstone.Credit: Nine

Images from the scene show multiple emergency vehicles by the business, which lies at an intersection adjacent to Riverstone station.

Local state MP Warren Kirby said he was deeply saddened by the tragic death, and asked NSW Health to provide him with immediate updates on the welfare of those hospitalised.

“My thoughts are with the family of the person who has died and with everyone affected, including first responders and their loved ones,” the Riverstone MP said.

Forensic examinations to determine the origin and nature of the leak are under way.

Forensic examinations to determine the origin and nature of the leak are under way.Credit: Nine News

“I will keep the community updated as more information comes to hand.”

Haveli is a restaurant chain with venues in Riverstone and Stanhope Gardens.