Hart was convicted in 2007 and was given an automatic life sentence, with no chance for parole for 25 years.
But he's been appealing the verdict ever since.
The Newfoundland and Labrador Supreme Court of Appeal gave two reasons for its decision.
The three-judge panel unanimously decided that Hart should have been allowed to testify in private, because of his tendency toward epileptic seizures and the stress of thinking and speaking clearly with many people around him.
The judges ruled that audio and video transmission of his testimony could have been provided to spectators outside the courtoom.
And in 2-1 decision, the appeal judges found that evidence of confessions obtained from Hart by police in a so-called “Mr. Big” sting was obtained by improper coercion and inducements, breaching Hart’s Charter rights.
The court ruled that the confessions should not have been admitted at trial.
Chief Justice Derek Green and Justice Michael Harrington concurred, with Justice Leo Barry dissenting.
The appeal court judges noted that the main evidence presented against Hart at trial was an admission to undercover police officers that he pushed his daughters into Gander Lake.
Hart was led to believe the officers were criminals, as a result of the “Mr. Big” operation launched by police.
Initially, the judges noted in their decision, Hart told police that his daughter Krista fell off a wharf and he panicked, leaving her twin sister Karen behind. He then drove 11 kilometres to his apartment in Gander - even though he had a cellphone in his car - to seek help from his wife.
When the couple returned, they found both children in the water. Rescuers who arrived later could not save either of the twin girls.
Hart later changed his story, telling police he had an epileptic seizure, and didn’t know how the girls ended up in the water.
In early 2005, the RCMP created what the appeal court judges called “an elaborate undercover operation,” recruiting Hart to join a “fictitious criminal organization.” The top boss, “Mr. Big,” would decide whether Hart could be trusted.
Undercover officers hired Hart, paying him $16,000 to simulate crimes involving fake credit cards, forged passports and fake casino chips. He was put up in luxurious hotels, and dined at fine restaurants.
In April 2005, Hart allegedly admitted killing his daughters. That admission was only recorded in the notes of police. Hart said they were wrong.
Two months later, after being pressed by the make-believe crime boss about what really happened, Hart was videotaped on the wharf re-enacting the drownings for “Mr. Big.”
Hart was convicted by a Newfoundland and Labrador Supreme Court jury on March 28, 2007.
He appealed, but opted not to participate in that process, believing it was part of the “Mr. Big” conspiracy against him.
'Mr. Big' errors
The majority decision of the appeal court concluded that the “Mr. Big” evidence should be tossed out.
“Mr. Hart was coerced into incriminating himself in circumstances where the reliability of the resulting confession is questionable,” noted Chief Justice Derek Green, writing for two of the three judges.
“This strikes at the core of the principle against self-incrimination, and strongly favours exclusion.”
During the original trial, the Crown noted that it likely would not have proceeded without the confession.
Hart’s unrecorded statement to undercover officers was not excluded by the appeal court decision.
The judges noted it's now up to the Crown whether to proceed with a new trial.
“Did Mr. Hart confess falsely — or truthfully?” Green wrote in the decision.
“We will never know with any degree of certainty or even assurance on the basis of the trial that occurred. In fact, without a truthful confession from Mr. Hart, we will not know whether a crime was committed at all.”