At most, the 24-bed facility, staffed 24 hours a day, has held four residents.
Corrections, under an Official Information Act request, released the costs of running the accommodation since its opening, which revealed a five-year high last year of more than $1.2 million.
After incurring significant set-up costs of more than $60,000 in its first year, spending on Matawhāiti has steadily increased since 2018.
An unannounced inspection by the Ombudsman's office in February 2020 highlighted a series of issues, most critically that residents were not given leave of absence from the residence for rehabilitative or humanitarian purposes, as legislation dictated.
"I consider that failure to provide further rehabilitation, on a permanent basis, would create a risk of arbitrary detention and could amount to cruel, inhuman or degrading treatment under ... the UN Convention against Torture and Other Cruel, Inhuman or Degrading Treatment," said chief ombudsman Peter Boshier.
Douglas took the fight to the High Court, seeking a declaration that the policy in place from January 2017 to March 2021 was unlawful.
Lawyers for the chief executive of Corrections accepted that the former protocols, which had since been amended, misinterpreted the legislation.
Previously, Matawhāiti residents were only allowed out to visit dying significant relatives, go to funerals, or for other "tragic personal circumstances".
Justice Jan-Marie Doogue issued the declaration and the court heard those on a public protection order now worked with staff to identify opportunities for leave, which could be centred around hobbies and interests or rehabilitative goals.
Douglas was jailed for three years in 2014 after targeting a 13-year-old boy through Facebook, changing his identity several times to evade the victim's suspicious mother.
He sexually abused the boy on two occasions and one psychologist said the predator simply did not have the ability to self-regulate his impulses.
Another said Douglas had a tendency to fixate on sexual activity "and is driven in his planning, manipulation and circumvention of barriers to offending".
His sex convictions stretched back to his teenage years and while at school, the court heard, he persisted in sexualised behaviour with other kids even with teachers present.
Behind bars, Douglas targeted young and vulnerable prisoners and was kicked out of specialist sex-offender treatment for his lack of progress.
Public protection orders are re-evaluated annually, and a mandatory High Court review of the continuation of the order is undertaken after five years.
Douglas' review was due to take place earlier this year but Corrections national commissioner Rachel Leota said there had been no decision reached.
"This process for the continuation of the order is currently ongoing," she said.