The autistic and epileptic 20-year-old had lived at a group home in Sydney's west for a few months when she slipped below the water in the bathtub and drowned last Thursday, her mother Tanya Petrus told AAP.

"She'd written all her birthday invitations," the grieving mother said through tears.

"I want to make sure no parents go through what I'm going through. This has changed me forever."

Ms Petrus, a single mother of six children, is demanding to know how her daughter could have been left alone to die.

She'd long been reluctant to ask for help caring for Merna, but did so after the 20-year-old's behaviour became increasingly difficult to manage.

Some mornings, Merna had bites on her lip and tongue, signs of overnight seizures.

Fearing her daughter might fall down the stairs or collapse in the shower, Ms Petrus decided she needed 24-hour care.

NDIS funding and meticulous research led her to Afford, which operates group homes in Sydney's west.

"In 20 years I never asked for respite or help ... this was the only time I asked for help," Ms Petrus said.

The single-storey, five bedroom brick home in Woodbine that Merna moved to in mid-March cost thousands of dollars each week under the NDIS.

Merna loved three things - Star Wars, swimming and routine - so it took time to accept the significant change in her living conditions.

After a rocky start, by late April Merna was warming to staff and making friends.

'I feel guilty, I kept saying to her 'stay, you'll have friends'," Ms Petrus said.

"I would see her once or twice a week. I kept asking (Afford) 'Please have regular staff check her so she knows their faces'."

The last time she spoke to Merna was on the phone on the morning of May 23. They chose Sizzler for their next mother-daughter date.

The young woman had a medical episode at some point that afternoon, Afford's executive manager of client services, Casey Hailes, told AAP.

It's unclear what the nature of that episode was, but Merna was later allowed to take a bath.

Police were called about 7pm after she was found unconscious. Paramedics were unable to save her.

"Inquiries suggest drowning was the woman's cause of death," NSW Police told AAP on Thursday. "A report will be prepared for the coroner."

Ms Hailes says Afford is yet to be provided with a formal cause of death.

"I don't believe she was left in the bath for too long and I don't believe she drowned," she said, adding the event was "tragic".

"She was a very, very independent young woman."

The group home is staffed 24 hours a day with staff sleeping in a bedroom.

Ms Hailes said the home was "without a doubt" staffed correctly and with sufficiently experienced employees last Thursday.

But, Ms Hailes said, Afford had not been made aware of Merna's overnight seizures and, if that information had been disclosed, they would have submitted documents to the NDIS and applied for funding to properly meet those needs.

Afford has hired an external investigator to examine the incident which has been reported to the NDIS Quality and Safeguards Commission.

Staff were formally interviewed by police and have been offered face-to-face counselling.

Merna will be laid to rest next week with Star Wars-themed music playing at the church.

Australian Associated Press