Law and order spokesman David Clendon said it was "unconstitutional" and would breach the rights of people who have served the time for their crime.
But Ms Collins said not all prisoners could be rehabilitated and she wanted to push the law through, even if it was found to breach the Bill of Rights Act.
"We're not going to sit by and wait for people to come out of prison ... who have refused to undertake rehabilitation and have indicated they might commit further atrocities."
The detention orders would mainly apply to child sex offenders who were sentenced before preventive detention - which often means a life sentence unless the prisoner can show they are no longer a risk - was an option.
Ms Collins did not want to name cases, but said in general there have been times when Corrections had no choice but to release prisoners, despite threats to re-offend. "And they have gone on to rape and kill.
"If we consider we have civil detention for people under the Mental Health Act, it seems only right that we do what we can to prevent our most vulnerable from being abused in this way."
There have been several high-profile serial offenders, including Malcolm Chaston, who in 1989 raped a woman at knifepoint and was jailed for eight years. In 2002, Chaston was jailed for six years for bashing, choking, and sexually violating a woman he knew. In 2010, he murdered Christchurch woman Vanessa Pickering.
Stewart Murray Wilson, dubbed the "Beast of Blenheim", was jailed in 1996 for offences spanning 25 years, including rape, attempted rape, indecent assault, wilfully ill-treating a child and bestiality.
Ms Collins said Corrections had estimated that between five and 12 offenders would be in civil detention over a 10-year period. The High Court test would be very high, she said.
"We're talking about a high imminent risk of serious violent and sexual offending. There are some people for whom rehabilitation does not work."
The centre would be secure and likely be on prison grounds. It could be similar to low-security accommodation for prisoners who were due to be released, although Ms Collins said that would not be suitable for violent offenders.
The facility would cost $400,000 to $700,000 per bed, but would be offset by the cost of having them on round-the-clock extensive supervision, which costs $300,000 to $400,000 a year per offender.
It would be funded through existing baselines, which was possible because the projected prison muster was falling.
National also announced plans to introduce pre-parole screening to weed out prisoners unlikely to be granted parole, increasing penalties for child pornography offences and introducing random drug and alcohol testing for those on community sentences or bail.
Detention
National's law and order policy:
* Civil detention for the worst sexual and violent offenders at imminent risk of re-offending. They would be detained indefinitely until the Parole Board was satisfied they were no longer a safety risk.
* Corrections estimates it would apply to five to 12 offenders over a 10-year period, mainly child sex offenders.
