A man and woman have been found dead inside a western Sydney home, sparking a Homicide Squad investigation into a possible murder-suicide.

A relative is understood to have found the pair deceased in their home on The Horsley Drive, Smithfield, shortly after 12.30pm on Sunday.

Also in the home was a four-year-old girl, who was asleep when the relative arrived.

The pair have not been formally identified.

Police are treating the deaths as suspicious.

Homicide Squad detectives and police from Fairfield police station, are working to determine what has occurred.