The Department of Corrections is still to use a new law designed to protect the public from high-risk offenders.
Public Protection Orders (PPOs) came into law in December 2014 and allow high-risk criminals to live on prison grounds rather than in the community after they complete their sentence. Corrections has applied for a PPO three times. None has been granted.
Acting deputy national commissioner Darius Fagan acknowledged how difficult it was to get a PPO enforced.
"Only a very small number of people are likely to become subject to public protection orders, and the threshold is very high."
Labour's Corrections spokesman, Kelvin Davis, said if the legislation couldn't be used on a high-risk rapist, as in the latest case, it was time the law was reviewed.
"Public safety is paramount and this legislation needs to looked at. We need to go back and fix it so that the worst offenders are subject to PPOs."
Corrections Minister Judith Collins said the legislation did not authorise her to get involved in individual cases. "However, in general I would expect the department to apply for a PPO in cases where it is appropriate, and likewise to consider appealing unsuccessful applications, if appropriate in the circumstances and in line with legal advice."
A spokesman for Justice Minister Amy Adams said it was "very early" to be evaluate the law's efficacy.
"When Parliament passed the Public Safety (Public Protection Orders) Act 2014 it was clearly signalled that the orders were designed to protect the public from a very small number of offenders who are exceptional in that they pose an imminent and high risk of serious sexual or violent offending," Adams said.
"It's very early to be forming conclusions about whether it is effective or fulfilling the policy intent."