Justice Minister Judith Collins is scrambling to push through a new law aimed at keeping a handful of high-risk sexual offenders behind bars, including the rapist known as the "Beast of Blenheim'', who will otherwise be released in five months.
But a legal expert says National's attempts to create a "civil detention'' regime could face challenges on human rights grounds.
If the legislation is to be ready in time to prevent the release of Blenheim rapist Stewart Murray Wilson, the government could also have to shortcut the normal legislative steps, something that would be "bad parliamentary process'', Otago University law professor Andrew Geddis said.
Wilson has served 18 years of a 21-year sentence for a catalogue of sexual and violent crimes against women and girls over the two decades leading up to his sentencing in 1996.
As he was sentenced under old laws, Wilson cannot be kept in prison beyond September 1. By contrast, an offender who committed the same crimes today could be sentenced to "preventive detention'', with little or no prospect of ever leaving prison.
Last November Prime Minister John Key and then-Corrections Minister Collins unveiled a new "civil detention'' regime that would allow prisoners such as Wilson to be held beyond the end of their sentence, on the grounds of public safety.
Collins told the Sunday Star-Times the legislation was under development as a stand alone bill, and would be introduced to the house "as soon as possible''.
She rejected any suggestion concerns over human rights were delaying work on the bill, and said the draft legislation would be "assessed for consistency with the human rights affirmed in the New Zealand Bill of Rights Act 1990''.
Collins would not be drawn on whether the legislation could be enacted swiftly enough to prevent Wilson's release.
She said the bill's "public protection orders'' weren't designed to target a specific individual, and were expected to apply to "between five and 12 offenders over a 10-year period, the majority of them child sex offenders''.
Colin MacKay, the retired Blenheim detective who led the 18-month investigation into Wilson's crimes, told the Star-Times he feared Wilson was at high risk of reoffending.
He said he was a great believer in the philosophy that once a prisoner had served their sentence they should be able to put their past behind them, but Wilson was "an exception'', whose offending took place over decades, and who had never accepted any responsibility for his crimes.
Wilson's crimes came to light after he was approached by a current affairs reporter to discuss the death of his infant son, Mervyn. The brain-damaged baby had been born prematurely on the day Wilson beat his de facto wife unconscious.
A television producer reported Wilson to the Children and Young Persons Service after learning his daughter did not attend school, and appeared to be poorly looked after.
Police became involved soon after.