Sydney police say one person is dead and three others seriously injured after apparently inhaling nitrous oxide, commonly known as "laughing gas," in a car.

Early Sunday morning, in the suburb of Toongabbie, four people were found unconscious inside the car. It is reported the man who stumbled on the scene ran to the nearest house for help, screaming the car was full of gas.

Police say a 38-year-old man died, a 23-year-old woman was in serious condition, while two more men were stable. A gas cylinder discovered in the car was labeled "nitrous oxide."

Police inspector Damon Cox says confirmation of the contents will not come until a thorough examination is conducted.

Cox adds, "People do some weird and wondrous things to try and get a bit of a buzz. So whether these people were sort of using it to maybe get a bit of a high or something like that. It obviously had disastrous effects."