Prime Minister John Key says it would be really challenging to pass retrospective law to stop the release of sex offender Stewart Murray Wilson.
Wilson, 65, labelled "the Beast of Blenheim", is due to be released on Wednesday after completing a sentence handed down in 1996 for 22 sex offences over 25 years. He has been moved to Whanganui Prison ahead of the release.
His 17 parole conditions are reportedly the strictest in New Zealand history.
Wilson will seek less stringent conditions in the High Court at Wellington on Monday while Whanganui District Council is challenging the release in the same court.
Mr Key told TVNZ's Breakfast programme that he understood the anxiety in Whanganui about the pending release but said "he's got to come out".
He said the government was changing the law so that in extreme cases prisoners would continue to be detained by a civil detention order.
He reiterated that the release conditions for Wilson were extremely stringent.
"Is he going to be in much different condition from being on one side of the wire to the other, arguably not."
Mr Key said it was possible to pass retrospective legislation but "generally it is seen as not a good thing".
"So it's a bit of a tough call to go and do that."
NZN