Sex offender Stewart Murray Wilson could have been detained indefinitely if the government hadn't been slow to introduce legislation allowing that to happen, Labour says.

Justice Minister Judith Collins on Tuesday brought in a bill enabling the courts to keep high-risk offenders in jail beyond their original sentence.

It has been introduced less than a month after the man dubbed the "Beast of Blenheim" was released from prison to a self-care unit on Whanganui Prison grounds, where he will serve his parole.

Wilson had completed a sentence handed down in 1996 for 22 sex offences against women, children and animals over 25 years and could not legally be kept in prison any longer.

Labour's justice spokesman, Charles Chauvel, says the government promised the indefinite detention law a year ago.

"We heard nothing from Judith Collins until the introduction of the legislation today," he said.

"In the interim, one of the five to 12 offenders it specifically targets - Stewart Murray Wilson - has already been released."

Mr Chauvel says others could be released while the Public Safety (Public Protection Orders) Bill goes through its legislative process.

Ms Collins says her bill will allow the High Court to order the detention of offenders who pose a very high risk of imminent and serious sexual or violent offending following their release from jail.

She says the orders are expected to apply to between five and 12 offenders over the next decade, many of whom will be child sex offenders.