Power has been partially restored to Mt Eden prison but the site remains in lockdown.
The power cut happened about 8.30am and has meant that 12 cases were unable to go ahead as scheduled in court.
A Ministry of Justice spokeswoman said 10 cases were adjourned at Waitakere District Court and two cases were adjourned at North Shore District Court.
Acting prison director Stephen Parr said the power outage had impacted some operations but the site had remained secure and locked down.
All of the prisoners had been accounted for, he said.