Stricken police hailed in restaurant gas leak tragedy

"Brave" police officers risked their lives by trying to save a young man from a deadly gas leak at a restaurant.

The 25-year-old man died and six others, including the police officers, are in hospital after a gas leak at the Sydney business on Tuesday.

The officers tried to revive the man, administering CPR amid extremely high levels of carbon monoxide, NSW Police Assistant Commissioner Gavin Wood said.

He described the officers who were taken to hospital as "extremely brave".

"I want to acknowledge the police who put their lives at risk going into the environment, commencing CPR, trying to save this young man's life and now they are themselves subject to medical intervention," Mr Wood said.

Five officers and another person were taken to hospital after going to the site in Sydney's northwest.

"It's a tragedy in terms of loss of life," Mr Wood told reporters.

"Early indications are he (the deceased) may well have been employed within that restaurant environment."

NSW Ambulance crews including intensive and critical care paramedics responded, transporting seven patients to hospital, a spokesman told AAP.

The patients were in a stable condition.

A Fire and Rescue NSW official said crews worked with police to remove a number of occupants from the ground floor and level two of the building.

The young man was found dead about 9.15am after emergency services responded to reports of the gas leak on Garfield Road at Riverstone.

Hazmat crews and ambulances could be seen parked out the front of the premises as investigations into the leak continued.

Fire and Rescue NSW said staff were on the scene for much of Tuesday as the investigation continued.

"Hazmat specialists are also carrying a suite of gas and atmospheric detectors, taking air samples to measure air quality and to determine the cause of the incident," a spokesperson said.

Local MP Warren Kirby said he was saddened by the death."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 on social media.

"I've received advice that there is no ongoing risk to the wider community, with an exclusion zone established by Fire and Rescue around the site."