Two child killers jailed for brutal attacks on young girls have been denied parole for three years - the toughest sanction the Parole Board can impose.

Jules Mikus and Luke Frederick Sibley, who are serving sentences of life imprisonment for unrelated but equally horrific crimes, were both declined parole when they became eligible in February.

Now the Parole Board has imposed its toughest sanction by denying the men the right to apply for parole for three years, unless their circumstances change significantly.

Offenders who are eligible for parole are usually entitled to annual parole hearings, but the board can make three year postponement orders in cases where their circumstances are unlikely to change.

Mikus, 53, was sentenced to life imprisonment and preventive detention in 2002 after he was found guilty of raping and killing six-year-old Napier schoolgirl Teresa Cormack in 1987.

He was due to undergo a programme for child sex offenders next year but continues to deny his guilt, which the Parole Board said was a considerable impasse to treatment.

It declined parole because Mikus was effectively refusing treatment and was of high risk of reoffending.

Sibley, 33, was sentenced to life imprisonment in 1997 after he strangled and suffocated three-year-old Christchurch girl Brittany Crothall to death and tried to kill her mother with a hammer.

The Parole Board declined his release because it was "essential'' he take part in a programme for child sex offenders, which he was due to start next July.

It noted that although Sibley was not charged with sex offences, it had been "part of his murder'' and there was strong evidence he was a sexual offender.

Both Sibley and Mikus did not attend hearings this month to consider postponement of their parole eligibility.

The board's decisions from those hearings, released to APNZ today, said both men would "plainly not be suitable for release'' at their scheduled hearings next February.

The board imposed a postponement order on Mikus until January 2015 and on Sibley until February 2015.

Sibley, who killed the toddler as an 18-year-old, will have spent half his life in prison by the time he is next eligible for parole.

Both men would be able to apply for parole earlier if there was a significant change in their circumstances.

The board noted both offenders were well behaved low-medium security prisoner who kept themselves busy in the structured prison environment.

- APNZ