The power cut impacted the emergency department and other areas in the hospital nearby, he said.
Ambulances diverted patients to other hospitals across the city for four hours.
"Power has now been restored and there were no adverse patient outcomes," a spokesperson said.
Firefighters had to supply emergency lighting to the hospital.
A Fire and Emergency NZ communications shift manager confirmed a unit was sent to North Shore Hospital to provide lighting at 1.30am.
St John Auckland district operations manager Glenn Metcalfe said two additional ambulance crews were deployed to help last night after they were notified of a power outage affecting the emergency department and other acute facilities at North Shore Hospital.
Ambulance crews worked with the Auckland District Health Boards to take patients to other hospital emergency departments across the city between 11pm Sunday and just after 3am this morning, he said.
An Auckland DHB spokesperson said a very small number of patients were diverted to Auckland City Hospital due to the power outage.
"We put in place contingency plans in case they were needed, but overall there was minimal impact on Auckland City Hospital."