A man is in custody after three people were killed in a suspected domestic violence incident at a suburban home in a quiet cul-de-sac.

Police arrived at the house at Rosemeadow, in Sydney's southwest, about 1.30am on Sunday after reports of a disturbance.

Officers found a 64-year-old man suffering serious head injuries at the property, along with the bodies of a woman in her 60s and a man in his 20s.

The injured man received medical treatment from NSW Ambulance paramedics and Liverpool Hospital, but he later died.

The victims "were assaulted by a man known to them", according to police.

A man in his 30s was arrested after arriving at the scene in a silver sedan about an hour later.

Police arrested the 32-year-old and he was taken to Campbelltown police station for questioning.

No charges have been laid.

Officers set up a crime scene and numerous police were still on-site later in the morning.

A report will also be prepared for the coroner.