DNA evidence later revealed Jules Mikus, now 53, as six-year-old Teresa's murderer. He was convicted and sent to prison in 2002.
Mansfield wrote to the Ministry of Justice in December - in a letter provided to the Herald on Sunday by Montaperto - to state that he believed a "significant miscarriage of justice has occurred".
The most startling claim was an approach from a man who claimed to be a member of the jury which had convicted Montaperto in 1988.
The man came forward after a Herald on Sunday story about the case.
Mansfield said he was careful to not speak to the juror. He instead had an independent lawyer interview him and prepare an affidavit.
He said the juror claimed a police officer visited an employer of one of the jurors during the kidnapping trial.
The employer was told Montaperto was "the primary suspect or stronger in the murder of Teresa Cormack".
"I also understand that the juror then conveyed this to the full jury and that this information influenced their verdict."
Mansfield said he could not verify the claim but "it appears genuine and I must act on it".
He said the affidavit would be filed with the appeal and likely lead to an investigation by police or the Independent Police Conduct Authority.
"This will be a serious matter and any party identified, if convicted now, would be imprisoned."
Mansfield said he had hired an investigator who confirmed Montaperto had a "provable" and "watertight" alibi for his whereabouts at the time of the kidnapping.
He said new evidence had also emerged in the form of "key documents ... which directly contradict the Crown case".
Mansfield said the case raised "important constitutional issues" which included historic police conduct, the "extremely rare and sensitive issue of juror contact" and the conviction of an innocent man.
He said the case was "seriously concerning" and it appeared the only reason Montaperto was arrested, tried and convicted in the kidnapping case was because of the fabricated link to the Cormack murder.
Montaperto, who has always maintained his innocence in the kidnapping case, said he was desperate to have his name cleared.
"I knew I was wrongly convicted and I've battled it for years and years but no one believed me."
He said being linked to the murder of Teresa had ruined job prospects, set him against police and led to the assault which almost killed him.
"There's evidence to overturn my conviction. How long do I have to wait?"
Senior officers who led the police inquiry into the murder were astonished at the claim.
The inquiry leader when Mikus was convicted, Brian Schaab, said police were aware of the seriousness of approaching jurors.
Schaab, who retired from the police on Friday after 44 years, said: "It doesn't sound like that happened but I don't know. It would be a cause of major concern if it did happen."
Alan Aitken, the inquiry leader immediately following the murder, was also stunned.
"I don't believe it for a second. There was nothing like that which was even dreamed of.
"If it is proven true then some member of the police or former member of the police would find themselves in jail."
A police spokeswoman said serious allegations against police - current or former - would be investigated.
A Ministry of Justice spokeswoman refused to discuss Mansfield's letter.
She said Montaperto could approach the Solicitor General if he believed there was a miscarriage of justice.
