Police have launched an investigation into the death of a seven-month-old baby girl after finding adult medication in her stomach.

The Morrinsville girl died in March in what was initially believed to be a case of Sudden Infant Death Syndrome

But concerns were raised when ESR toxicology results revealed traces of the adult medication in her system.

Detective Senior Sergeant Karl Thornton said the girl's 22-month-old sister was also treated in hospital last month after it was suspected she consumed medication.

"We're focussing on how the seven-month-old ingested the medication.

"We're trying to confirm if the older sister swallowed similar pills. At this stage we are keeping an open mind as to what might have happened."

The two girls were living with their mother and an older sibling at the time of the incidents.

Both the 22-month-old and the older sibling have been placed in the care of Child, Youth and Family for the course of the police investigation.

- NZ Herald staff