"It doesn't matter whether you agree with [euthanasia] or not. It's illegal," prosecutor Paul Kerr told a Goulburn jury on Thursday during his closing address in the NSW Supreme Court trial of Barbara Eckersley.

Mrs Eckersley, 69, has admitted putting drugs including so-called "green dream", which is used to euthanise animals, into meals and feeding them to her 92-year-old mother, Dr Mary White, at the Warrigal aged care facility in Bundanoon.

But she has pleaded not guilty to murder, with her defence claiming she did not intend to kill the acclaimed former Canberra scientist, that the drugs may not have been a substantial or significant cause of Dr White's death, and that she was mentally impaired at the relevant time in August 2018.

Mr Kerr argued on Thursday, however, that Mrs Eckersley had reached "the end of her tether" and made a conscious decision to end Dr White's life before doing exactly that.

"She could not bear to see her mother continue, as she perceived it, in pain and suffering," he claimed, saying that sometimes "good people do bad things".

Mr Kerr said Mrs Eckersley's comments to police a few days after the death, when she said she had expected her actions to end her mother's life, accurately reflected her state of mind when she put the drugs in Dr White's food.

He told the jury that Mrs Eckersley's evidence from the witness stand that she had been confused, and had in fact only intended to sedate a suffering Dr White to a level of comfort, should be rejected.

"The Crown says that what Barbara Eckersley is really doing is telling you anything at all she thinks will help her convince you that she is not responsible for her mother's death," Mr Kerr said.

The prosecutor went on to say he expected expert evidence would leave the jury in no doubt that the pentobarbital, or "green dream", had been a substantial or significant cause of Dr White's death.

He also rubbished Mrs Eckersley's claims that she administered the drugs during "out-of-body" experiences, then forgot for days about having done so, calling these assertions "nonsense".

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Turning to the topic of euthanasia, Mr Kerr said there was evidence Mrs Ecksersley's husband, Richard, was an advocate for assisted dying.

He said Dr White's former doctor had testified that Mr Eckersley once asked him: "Are you not able to arrange for euthanasia for Mary?"

Mr Kerr said Mrs Eckersley, who denied that this ever happened, was present at the time and had told police days after Dr White's death that she wanted to "confess" because it might otherwise look like her husband had been responsible.

He told the jury that the 69-year-old had been concerned about this because "she knew that what she had done was effectively to have euthanised her mother".

Early in his closing remarks, defence barrister Kieran Ginges said the jury would not be satisfied that Mrs Eckersley had intended to kill her mother, or that she had even caused Dr White's death by giving her non-prescribed drugs.

He said an experienced forensic pathologist had given evidence that many things might have killed Dr White, who had been in such poor health that "she could have died at any time".

Mr Ginges also said Mrs Eckersley had been a "credible, reliable, impressive and honest witness" when she gave evidence in court and insisted she had only been attempting to relieve her mother's pain.

Arguing that Mrs Eckersley had been suffering from "an abnormality of mind" at the relevant time, he pointed to her description of having effectively been "watching herself from a displaced and emotionally disconnected place".

Mr Ginges will continue his closing address on Friday.

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