Labour attempted in 2018 to repeal the three-strikes law, which was put in place by the previous National government in 2010, but the effort was stymied by New Zealand First.
For the past three years, Fitzgerald's sentencing appeal has made its way through the courts. The Court of Appeal upheld the sentence in July 2020, despite judges acknowledging it was "disproportionately severe" and breached his rights.
Justices David Goddard and Denis Clifford wrote last year that they tried to "identify a tenable reading" of the law that would allow the mandatory sentence to be avoided.
"We have reluctantly concluded that this course is not open to us," they said.
The Supreme Court issued its 117-page ruling on Thursday afternoon.
Four of the justices agreed the sentence was severe enough it breached the Bill of Rights, which states that "everyone has the right not to be subjected to torture or to cruel, degrading, or disproportionately severe treatment or punishment". Parliament didn't intend for the three-strikes law to result in sentences that violate the Bill of Rights or New Zealand's international human rights obligations, the justices found.
"The appellant's sentence of seven years' imprisonment went well beyond excessive punishment and would shock the conscience of properly informed New Zealanders," the justices said.
Fitzgerald's lawyers also argued to the Supreme Court that he should be discharged without conviction altogether due to the Bill of Rights, but the justices unanimously denied the request.
Chief Justice Helen Winkelmann said, however, that discharge without conviction should be available to some third-strike offenders on rare occasions.
"This case comes very close to being one in which a discharge ... would be appropriate – indeed, it might have been such a case, were it not for public safety concerns," she wrote.
But a conviction will allow him access to mental health services that can address public safety concerns, she said.
The law was intended to apply to serious violent and sexual crimes, 40 of which were outlined in the legislation. Judges were instructed by lawmakers to give a formal warning after the first offence and order a sentence without parole for the second. The third conviction results in the maximum sentence - without parole if the judge doesn't find such an order to be "manifestly unjust".
In Fitzgerald's case, the High Court judge who initially sentenced him had already declined to make him serve the term without the possibility of parole.