An Australian woman who spent 20 years in prison for killing her four kids was pardoned and freed Monday after a scientific breakthrough showed they likely died of natural causes.

Kathleen Folbigg, now 55, had long been condemned as Australia’s worst female serial killer, sentenced in 2003 to 40 years in prison for the murder of three of her kids and the manslaughter of the fourth.

She was convicted largely on how seemingly inconceivable it was for all four — aged between 19 days and 19 months — to have died in similar circumstances over a decade from 1989.

However, scientific breakthroughs showed that her daughters — Sarah, 10 months, and Laura, 19 months — actually had a rare genetic variant, CALM2, while a son, Patrick, 8 months, had an underlying neurogenic disorder.

That suggested that all three likely died of natural causes, as their mom has always maintained, former chief justice Thomas Bathurst concluded in a review started last year. It also undermined the manslaughter case, in which prosecutors initially argued that she must have been responsible for the death of 19-day-old Caleb in 1989 if she later killed the other three.

The genetic variant “is a reasonably possible cause” in the deaths of Sarah and Laura, lawyer Sophie Callan said expert evidence in the fields of cardiology and genetics found. Another “reasonably possible cause” of Laura’s death could be myocarditis, an inflammation of the heart, Callan said.

And for Patrick, there was “persuasive expert evidence that as a matter of reasonable possibility, an underlying neurogenetic disorder” caused his sudden death, the lawyer added.

Some 90 scientists, medical practitioners and experts signed a petition saying a second inquiry by Bathurst was “based on significant positive evidence of natural causes of death.”

Bathurst concluded that there was nothing to show that Folbigg “was anything but a caring mother for her children.”

New South Wales Attorney-General Michael Daley freed Folbigg on Monday with an unconditional pardon.

“There is a reasonable doubt as to Ms. Folbigg’s guilt of the manslaughter of her child Caleb, the infliction of grievous bodily harm on her child Patrick and the murder of her children Patrick, Sarah and Laura,” Daley told reporters.

“I have reached a view that there is reasonable doubt as to the guilt of Ms. Folbigg of those offenses.”

After walking free, Folbigg was filmed by 9News emotionally hugging childhood friend Tracy Chapman.

“I am so elated it’s not funny,” Folbigg gasped, telling the outlet she could not think of anything beyond getting “a drink of water.”

“I am nervous and I’m everything,” she said, also gasping while hugging her friend: “I’m still, like, ‘Is this happening?’ I’m like, ‘Oh my God!'”

Folbigg’s initial 40-year sentence was later reduced to 30 years, which would have lasted until 2033. She was not eligible for parole for another 5 years.

While the unconditional pardon allowed Folbigg to walk free, it does not overturn her convictions. That would need to be considered later by the state Court of Appeals.

A government spokesperson said Folbigg will pursue compensation for the “20 years of her life that has been lost,” the Guardian reported.

Daley said he pre-warned Craig Folbigg, the babies’ father who still believes his ex murdered their children, his lawyer told 9News.

“It will be a tough day for him,” Daley said of the dad who helped build the case against his ex.

“Given all that has happened over the last 20 years, it is impossible not to feel sympathy for Kathleen and Craig Folbigg.”

Daley later told CNN:  “We’ve got four little bubbas who are dead. We’ve got a husband and wife who lost each other. A woman who spent 20 years in jail, and a family that never had a chance. You’d not be human if you didn’t feel something.”

The dad still maintains that Folbigg’s diaries contained admissions of guilt.

However, in his review report, Bathurst said the “evidence suggests they were the writings of a grieving and possibly depressed mother, blaming herself for the death of each child, as distinct from admissions that she murdered or otherwise harmed them.”

With Post wires