The 68-year-old woman was detained on Thursday, with detectives at the same time conducting a search on her home.

Detective Superintendent Des Bray said the arrest brought to a conclusion the initial investigation which had centred on the care Ms Smith received.

He said it was alleged she died of serious criminal neglect and her death was preventable.

"It's been a comprehensive investigation to date, looking at all aspects of the care provided to her," he said.

"We've been able to build a picture of Annie's life since the death of her parents in 2009.

"I can tell you that between 2009 and 2018 Annie led a more active and outgoing life.

"Between 2018 and 2020 she became increasingly socially isolated and the quality of her life deteriorated rapidly."

Ms Smith, who suffered from cerebral palsy, died in hospital in April from septic shock, multiple organ failure, severe pressure sores and malnourishment while under the care of the National Disability Insurance Scheme.

Supt Bray said police would also continue to develop a comprehensive brief ahead of a coronial inquiry and were still investigating the dead woman's financial affairs.

Detectives were also cooperating with other investigations, including an independent inquiry by former Federal Court judge Alan Robertston, who was appointed by the NDIS commissioner to look into the case.

At the time of her death, police said Ms Smith had been spending her days and sleeping at night in the same woven cane chair, with extremely poor personal hygiene and no nutritional food.

After Thursday's arrest, Premier Steven Marshall said it was important to be "seeing justice being done in what could only be described as a tragic, tragic case".

"What we know is there has been an arrest today, there have been charges laid and there will be a person appear before the court," he said.

The woman was expected to front Adelaide Magistrates Court on Friday.

Australian Associated Press