Nancy Beaumont, the mother of the three missing Beaumont children, has died in Adelaide aged 92.
Mrs Beaumont passed away on Monday in a nursing home and her death was confirmed in a notice published on Thursday.
The disappearance of her three children - Jane, 9, Arnna, 7, and Grant, 4 - on Australia Day in 1966 is one of the country's most enduring mysteries.
The children never returned after leaving their parents' home for an afternoon at Glenelg Beach. There was an intense search but they were never found.
In 2013, investigators scoured a factory west of Adelaide, after two brothers told police they had spent the 1966 Australia Day weekend digging a large hole on the site at the request of the owner Harry Phipps.
Phipps died in 2014 but his son, who accused his father of sexual abuse, believed he was linked to the missing children.
The factory site was excavated by police in early 2018 but only animal bones were found.
Officer in charge of Major Crime, detective superintendent Des Bray, said police were still investigating Phipps and remain committed to solving the mystery.
"We will always do anything humanly possible to locate the Beaumont children and take them home to their family," he said after the search wound up.
"I don't think there's anybody in the country who doesn't want to find the Beaumont children."
A police statement released after Mrs Beaumont's death noted her passing "with sadness".
"Consistent with family wishes, SAPOL have no comment to make about her passing beyond expressing our sympathy," it read.
The disappearance of the Beaumont children has been one of Australia's most notorious cold cases and subject to wild speculation at times, including possible sightings of the trio living as adults overseas.
The children's father, Jim Beaumont, is also aged in his 90s and living in Adelaide.
Mrs Beaumont's death notice, published in The Advertiser on Thursday, said she would be cremated in a private ceremony.