Waikato police are investigating the death of a baby girl from Morrinsville who police say had traces of adult medication in her stomach.
Police say the seven-month-old child died in March this year in what was initially believed to be a case of Sudden Infant Death Syndrome.
Concerns were raised when ESR toxicology results revealed traces of adult medication in her stomach, they say.
In late April, the girl's 22-month-old sister was treated in hospital after it was suspected that she had also ingested medication.
Detective Senior Sergeant Karl Thornton said police are focusing on how the seven-month-old ingested the medication and trying to confirm whether her older sister swallowed similar pills.
"At this stage we are keeping an open mind as to what might have happened.
"We are not in a position to confirm any details about either incident at the moment," a statement from Detective Senior Sergeant Thornton said.
The two girls were living with their mother and an older sibling at the time of the incidents.
As an interim measure, the 22-month-old and the older sibling have been placed in the care of Child, Youth and Family for the course of the investigation.
Child, Youth and Family says it was not involved with the family prior to this.