A three-year-old boy has died from stab wounds and a man is in a critical condition after they were found by police at a unit in Sydney’s south-west.

Emergency services attended the unit complex on Washington Avenue, Riverwood, about 4pm on Wednesday.

Police located the dead child and a 45-year-old man with serious injuries after responding to concerns for their welfare.

The man was treated at the scene by paramedics and taken to hospital in a critical condition.

Police have established a crime scene and commenced an investigation into the circumstances surrounding the incident.

Anyone with information has been urged to contact police or Crime Stoppers.