The NSW Supreme Court jury on Thursday acquitted Barbara Eckersley of murder, but found the 69-year-old guilty of the alternative offence of manslaughter.

Eckersley was on trial in Goulburn for more than three weeks over the August 2018 death of her mother, the acclaimed former Canberra scientist Dr Mary White.

She admitted putting so-called "green dream" into Dr White's dinner on the evening of the 92-year-old's death at an aged care facility in the Southern Highlands, but she denied intending to kill.

Eckersley told police in the days after the incident that "I guess I was expecting her to die", but argued at trial that she had said the wrong thing while in shock.

Her case was that she had merely been trying to sedate her bedridden and often agitated mother to a level of comfort after losing faith in the medical regime at the Warrigal nursing home in Bundanoon.

Defence barrister Kieran Ginges also sought to cast doubt on whether the "green dream" drugs were a substantial or significant cause of Dr White's death.

He cited the evidence of forensic pathologist Professor Johan Duflou, who told the jury that Dr White's state of health was so poor "she could have died at any time" from a variety of ailments.

Mr Ginges also argued that Eckersley was mentally impaired by a major depressive disorder at the relevant time.

READ MORE:

But Crown prosecutor Paul Kerr told the jury in his closing address last week that what Eckersley had done "was effectively to have euthanised her mother".

He said Eckersley, frustrated by what she perceived to be substandard care at the aged care facility, had reached "the end of her tether" and made a conscious decision that it would be better for Dr White to die.

"It doesn't matter whether you agree with [euthanasia] or not. It's illegal," he told the jury.

Mr Kerr also said the cause of death had been made clear by Dr Rebecca Irvine, the forensic pathologist who performed the autopsy on Dr White.

Dr Irvine found "mixed drug toxicity" had killed the renowned environmentalist, with the drugs in question determined to be pentobarbital and benzodiazepine.

Pentobarbital is a barbiturate known as "green dream", which Eckersley had acquired while working as a wildlife carer in Canberra some 20 years earlier.

Eckersley also admitted that she had crushed sleeping tablets - benzodiazepines that were not prescribed to Dr White - and put them in her mother's dessert on an earlier occasion.

Mr Kerr said the Crown accepted that Eckersley was depressed about her mother's situation at the time in question, but he argued she was not impaired to the extent that she could not control her actions or distinguish right from wrong.

The jury retired on Monday afternoon and deliberated for nearly three days before returning its verdict.

Eckersley will remain on bail ahead of her sentencing, at which Mr Ginges has indicated he will seek a non-custodial sentence.

Justice Robert Beech-Jones will hear submissions from lawyers on Friday, ahead of a likely sentencing date on May 20.

Our journalists work hard to provide local, up-to-date news to the community. This is how you can continue to access our trusted content: