Two South Australian sisters have been denied their plea to feed their dying mother, with a public guardian refusing to reinstate the woman's feeding tube despite the intervention of the Catholic Church.

Key points: The OPA says several medical professionals agreed the tube was becoming burdensome on the patient

The OPA says several medical professionals agreed the tube was becoming burdensome on the patient SA's most senior member of the Catholic church wrote a letter of support for the sisters

SA's most senior member of the Catholic church wrote a letter of support for the sisters The elderly woman died this week after battling end-stage dementia and breast cancer

The Office of the Public Advocate (OPA) has been making decisions on behalf of the mother, after her daughters were stripped of her guardianship amid a dispute with her aged care provider.

The mother, who had end-stage dementia and breast cancer, died this week, more than a month after the nasogastric tube, which had been providing her nutrition, fell out.

Before her death, the sisters urged the public advocate to reinstate their mother's tube, arguing it would be in keeping with her Catholic faith.

But in a decision upheld by the state's Civil and Administrative Tribunal, the OPA declined.

Instead she was given saline through a drip.

"It's mum's religious belief and wishes, she always said, 'Where there's life there's hope' and she wanted to be fed and watered right until the end," the eldest daughter said.

In a letter to the family, the OPA said the tube would not be reinstated because the patient was in a "pre-terminal phase".

It said: "Treatments offered should not be those that prolong her life which would only result in more suffering."

"You just go in there and look at your mum and see her dying before your eyes and you're helpless, you can't do anything," the eldest daughter said.

"She's withering away and starving, you think, how did we get to this point? We can't honour your wishes, we can't give you your last bit of what you wanted in this life.

"We were forced to watch her body starve to death as she is jerking and shaking and gasping for breaths."

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Under South Australia's Palliative Care Act, a public guardian does not have to continue life-sustaining measures if the person is in a moribund or vegetative state with no prospect of recovery.

The woman's daughters argued their mother was not in vegetative state because she was aware of her surroundings and sometimes recognised people.

"Not long after the tube came out and she still follow[ed] us with her eyes and she cried," the younger daughter said.

"The OPA should be here watching this. They make decisions and step back. Why didn't they honour the fact she wanted to be fed and watered until the end?" the eldest daughter said.

Catholic bishop intervenes with letter of support

Before the woman's death, the Bishop of the Diocese of Port Pirie, Gregory O'Kelly, wrote to the OPA, throwing his support behind the daughters' fight.

Bishop O'Kelly is the most senior member of the Catholic church in South Australia, appointed by the Pope as the Apostolic Administrator of the Adelaide Archdiocese after the archbishop was convicted, and then acquitted, of concealing child sex abuse.

In the letter he said: "To deny food or drink, no matter how it is delivered, to such a person is to deny them ordinary means for sustaining life."

Bishop Greg O'Kelly wrote a letter of support in a bid to help the daughter's fight. ( Diocese of Port Pirie )

"There is a difference between sustaining the dying state and undertaking a deliberate withdrawal of treatment in order to bring about death," he wrote.

The family's priest for almost a decade, Father Paul Crotty said the woman was not in the final stages of life and had survived for more than a month on basic fluids.

"I think they've failed to understand the totality of this person and her underlying faith and values," Father Crotty said.

"I do feel sad, angry, frustrated that it's come to this. I couldn't believe it.

"I think the OPA has failed to be a voice, an advocate [for this person]."

Public advocate says feeding tube was burdensome

In a letter to the priest, the OPA said several medical professionals were consulted and agreed the nasal tube was becoming burdensome on the patient.

The OPA said there was a "difference of views as to what family members consider [the patient's] wishes would have been, were she not incapacitated".

The ABC has spoken to four of the woman's five children. Three of them supported the nasogastric tube being inserted and agreed on their mother's view to prolonging life.

The OPA said it couldn't comment on the case for privacy reasons, but said it must consider, based on evidence, what the wishes of the person incapacitated would be.

"It feels like because she's old and aged that it doesn't matter anymore … and elderly don't have a voice anymore," the older daughter said.

"As soon as the public advocate takes your parents, you've got not rights. You can try and talk to them, [but] it feels like you're not heard."