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.