A law rushed through Parliament on Thursday night has the potential to punish child-sex offenders twice, says the Green Party.
The Government says changes to the Parole Act were necessary to close loopholes and were a matter of public safety.
Previously the Parole Board could impose up to 10 years' supervision for convicted child-sex offenders, but could order electronically monitored home detention for only the first 12 months.
The changes mean the Parole Board can impose those conditions at any time throughout the 10-year period.
But Green MP Keith Locke says this essentially turns the supervision period into a second sentence.
The bill, supported by all parties except the Greens, passed through all its stages on Thursday night.
"The bill ensures that this is possible by clarifying that part-time residential restrictions can be imposed beyond the first 12 months following the offender's release from prison and for the remaining term of the extended supervision order," Justice Minister Simon Power told Parliament
The Green Party had agreed to support the bill until it saw criticisms from the attorney-general that it contravenes the Bill of Rights.
The law also requires the Parole Board to be satisfied that the offender will comply with the conditions of the supervision order before it can impose one.