A woman has had a conviction thrown out almost 30 years after she falsely admitted to killing her baby daughter.

In 1989 Terri Louise Friesen, then 34, pleaded guilty to the manslaughter of her 7-week-old baby, Chantelle.

The Court of Appeal today quashed Friesen's conviction.

Friesen falsely confessed to the crime to protect her then-partner, Brownie Walter Broughton. She was sentenced to six months' supervision.

Thirteen years after taking the fall for a crime she didn't commit, Friesen was proven innocent when Broughton walked into a Christchurch police station, admitted it was him and begged to be charged with manslaughter.

He confessed to shaking his infant daughter to death and letting his de facto partner take the blame.

Broughton said he had found God and wanted to atone for his sins.

He had earlier approached police in New Plymouth and tried to confess, but no action was taken.

Broughton was eventually convicted in relation to Chantelle's death - but Friesen's conviction was never expunged.

After Broughton confessed, Friesen told a court that she falsely admitted shaking the baby because she did not think the baby's father could handle spending time in jail.

She claimed the police also threatened that if one of them did not own up, her other daughter would be taken into Social Welfare care.

Chantelle died from non-accidental injuries as a result of being shaken.

A post-mortem examination revealed she had suffered from a brain injury and cracked ribs - an injury her mother says she knew nothing about.

In a 2001 interview with the Herald Friesen said she was seeking a pardon.

"I just want my name cleared," she said.

"I'm relieved that the truth is out there. I'm quite numb, it's pretty freaky."

The mother of seven described Broughton as a violent and evil man.

She claimed that at the time of Chantelle's death, police told her Broughton would die if he was sent to prison.

"He really looked like the broken one," she said.

"I was in a state of shock ... They looked at me as the hard one."