Police are working to determine why a substance was allegedly administered to a Sydney nursing home resident before she was found dead on Thursday night.

The 84-year-old woman’s body was found inside Our Lady of Consolation in Rooty Hill about 11.35pm.

WATCH THE VIDEO ABOVE: Daughter questioned over nursing home death

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Police allege a 53-year-old woman, the 84-year-old’s daughter, administered the substance.

“We don’t know exactly what that substance is at the moment, or whether that contributed to her passing last night,” NSW Police Detective Superintendent Darrin Batchelor said.

The woman was questioned by police on Friday but has been released without charge.

Staff at the nursing home, which has 300 elderly residents, called paramedics late on Thursday.

The daughter was “distressed when police arrived” and another family member — the late woman’s nine-year-old grandson — was also present at the nursing home but is now in the care of family.

The 53-year-old woman was taken to Mount Druitt Hospital for medical assessment.

Batchelor said she had no criminal history, and also noted that it is not believed she had any medical training either.

Elderly woman ill before death

The elderly woman “did suffer from a number of medical conditions”, the officer said.

“It’s really too early to determine what the motivation is at the moment, whether it was an assisted dying scenario, we really don’t know.

“The manner of the elderly woman’s death will be a matter for the Coroner to determine through toxicology reports.”

The aged care centre said it was “distressed by the death of our resident”.

“Our thoughts are with their family and loved ones at this time. We are continuing to work with the police,” it said in a statement.

A number of items have been seized from the nursing home, and CCTV footage is being reviewed.

Investigations continue.