A woman accused of murdering her elderly mother will argue the drugs she added to her mum's food were not the sole cause of death.
Barbara Eckersley, 69, is on trial in the NSW Supreme Court, charged with murdering her mother, renowned environmental scientist Mary White, at an aged-care centre in August 2018.
Eckersley denies killing her mother, who had severe dementia and other health conditions.
Ms White, 92, moved to the care home at Bundanoon - in the NSW Southern Highlands - from a granny flat in her daughter's backyard after suffering a stroke in 2016.
She was paralysed on the right side of her body and at the time of her death was incontinent, non-verbal and needed full-time care.
Eckersley was frustrated at the care provided to her mother and planned to move Ms White to a nursing home at Coffs Harbour where she would be closer to her other daughter.
The plan was in action and had been agreed between Eckersley and her siblings - sister Zoe and three brothers - who all gave evidence to the court in Goulburn on Wednesday.
Ms White, a great grandmother, was due to be moved on August 7 but she died two days prior, when Eckersley and her husband Richard were visiting.
Mr Eckersley said it was the shock of his life when days later his wife told him she had put a drug known as "green dream" into her mother's food on the day she died.
"I just couldn't believe it," he told the court.
Mr Eckersley said he wanted to wait until Ms White's toxicology report, but his wife was adamant about going to police.
After telling detectives she had put drugs in her mother's meal, Eckersley was told she would be charged with murder.
Her lawyer Kieran Ginges said Eckersley would dispute the drugs were the significant cause of Ms White's death.
She had a range of health problems including a heart condition and thrombosis.
"She could have died at any time," Mr Ginges told the court.
He also said Eckersley was severely depressed in August 2018 and did not have the capacity to understand the events.
Eckersley had the drugs left over from her time working as a wildlife carer in Canberra about two decades earlier, the court was told.
It was used to euthanise native animals.
She added a small amount to her mother's food thinking she would go to sleep, not that it would kill her, Mr Ginges said.
Crown prosecutor Paul Kerr said murder was usually associated with violence, mayhem, anger, revenge and jealousy.
He said that did not describe Eckersely, calling her a loving, caring and compassionate daughter, but that taking another life constitutes murder.
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Australian Associated Press