Dangerous child sex offenders are to be caged indefinitely in purpose-built pens behind the wire, after Cabinet gave the green light to new public protection orders.

The move comes as the Government yesterday introduced tough new bail laws to fulfil National's pre-election law and order pledges.

Justice Minister Judith Collins said Cabinet had signed off on public protection orders, and she expected to introduce related legislation to Parliament at the end of the year.

Under the proposals, authorities would apply to the High Court for an order – and offenders would be able to ask for a review.

The proposals would apply to a small number of "incredibly dangerous" sex offenders – estimated at between five and 12 in a decade – assessed as being at risk of sexual or violent reoffending.

Ms Collins said extended supervision orders – by which an offender on release would be monitored 24 hours a day – were not adequate.

"[Corrections] have to find a place in the community, the public don't want them there, they can be incredibly dangerous and very manipulative, particularly around young children," she said.

"It's too easy, in some cases, for things to go wrong."

Officials were working on proposals likely to see the offenders housed in "something like a flatting situation ... in one of our prisons where there is large amounts of land", she said.

"They will not be able to go out roaming around the countryside.

"Basically this is about keeping children safe from child sex offenders – the sort that once upon a time would have been committed to a forensic mental health unit."

Ms Collins admitted that housing the prisoners indefinitely would be expensive and costs were still being calculated.

"Money is not the big thing because these are people who will continue to be predators around children."

She dismissed reports the legislation was targeted at rapist Stewart Murray Wilson, the Beast of Blenheim, due to be freed in September.

The probation service has applied for an extended supervision order which will see him monitored for more than a decade.

Corrections Minister Anne Tolley was considering proposals to introduce a child sex offenders' register, after a convicted paedophile was found to be working as a teacher.

Labour's justice spokesman, Charles Chauvel, questioned the need for new legislation.

"I would have thought something like enhancing the extended supervision order, maybe providing for electronic monitoring, would be sufficient given we are talking about such a small number of people.

"But clearly that doesn't suit the Government's law and order agenda. They want a big hoopla about it ... they made a political decision during the election to make an announcement whether or not they thought there was a problem."

Ms Collins' Bail Amendment Act – to crack down on the high rate of offending on bail by defendants charged with murder, violent, sex and class A drug offences – yesterday passed its first reading at Parliament.

Attorney-General Chris Finlayson, speaking in the House on her behalf, said defendants charged with serious crimes would need to satisfy the court they will not commit violent offences while on bail.