An Australian mother jailed for killing her four young children has been pardoned after 20 years behind bars, authorities said Monday, after an inquiry upended the high-profile case.

Kathleen Folbigg was dubbed "Australia's worst female serial killer" after she was convicted in 2003 of murdering three of her children, and the manslaughter of a fourth.

Prosecutors argued she suffocated the children, who died between the ages of nine weeks and three years old, but Folbigg has steadfastly maintained each death was due to natural causes.

In 2021, dozens of scientists from Australia and abroad signed a petition calling for Folbigg's release, saying new forensic evidence suggested the unexplained deaths were actually linked to rare genetic mutations or congenital abnormalities.

New South Wales Attorney-General Michael Daley said a year-long inquiry, launched in May 2022, had now established "reasonable doubt" surrounding the convictions -- and that Folbigg had been pardoned.

"In the interests of justice, Kathleen Folbigg should be released from custody as soon as possible," he said on Monday.

In the absence of firm forensic evidence, prosecutors had argued that it was extremely unlikely four children would suddenly die without explanation.

But retired judge Tom Bathurst, who led the inquiry, said subsequent investigations found medical conditions that could account for three of the deaths.

Bathurst said Sarah and Laura Folbigg possessed a rare genetic mutation, while Patrick Folbigg may have had an "underlying neurogenic condition".

Given these factors, Bathurst found the death of Caleb Folbigg was also no longer suspicious.

He added that he was unable to accept that "Folbigg was anything but a caring mother for her children".

The Australian Academy of Science, which helped to spark the inquiry, said it was "relieved" to see justice for Folbigg.