Police are investigating the death of a man who is believed to have been assaulted and left outside a house on the New South Wales north coast.

Emergency services attended William Street at Tweed Heads South at around 3am on Sunday following reports of an assault.

Paramedics began treating a man they found lying on the ground outside a home but he died at the scene.

The man, believed to be in his 30s, has not yet been formally identified.

A crime scene has been established and the police are investigating.