A 5-year-old girl has died from a suspected methadone overdose in Sydney.

The child tested positive for the drug after being found unconscious two days ago.

Her grandfather raised the alarm after he tried to wake her at a home in Quakers Hill, in Sydney's north west, on Tuesday afternoon.

But the girl was non-responsive and having difficulty breathing, Hawkesbury duty officer Peter Jenkins said.

Advertisement

The man called triple-0 and began CPR on the little girl.

Insp Jenkins said vials of methadone were found at the house when emergency services arrived.

"One examination (at hospital) has revealed the child had methadone in her urine," Insp Jenkins said.

The Child Abuse Squad is investigating the incident.

Detectives reportedly last week interviewed neighbours and others who were at the house at the time, including another child.