Legislation enabling the so-called Public Protection Orders was introduced to Parliament on Tuesday by Justice Minister Judith Collins.
Ms Collins says the orders would apply to just a few of the most serious criminals, including serial sex offender Stewart Murray Wilson, who was recently released from jail to live on a unit on the Whanganui Prison grounds.
However, Wilson's lawyer Andrew McKenzie says his client will be very elderly by the time his extended supervision orders expire.
Mr McKenzie says the law change is an over-reaction to something that may not be a problem.
"What I've seen is a really intricate system which would appear to only apply to five to 10 people in the next decade, and it just seems a real over-reaction to something that's probably not even a problem."
Mr McKenzie says instead of a new law, the Government could consider making extended supervision orders more flexible.
Civil rights lawyer Michael Bott, QC, says locking up people indefinitely for crimes they have not yet committed breaches the Bill of Rights and international human rights conventions.
Mr Bott says judges can already order extended supervision and preventive detention, and the Government has not put up any evidence to justify the law change.
"Do you wish to respond to basically the smokescreen diversionary politics of people appealing to the base relevance of New Zealand society? Or do you wish to be known as a nation that upholds fundamental human rights and bring about legislation based upon actual figures and statistics and reason?
A high profile defence lawyer says a lack of resources could undermine the intention of a the law. Robert Lithgow, QC, says the bill talks about rehabilitation plans and opportunities for courses, but it does not mention resourcing.
"Their proposition is that it not be intended to be punitive, but to the person who is made subject to such an order it will seem like that."
Mr Lithgow says the danger is the regime will become indistinguishable from prison life.
But the Sensible Sentencing Trust says public safety must come before the rights of criminals.