A major power cut plunged parts of the North Shore Hospital into darkness for four hours last night, resulting in patients being taken to other hospitals further afield for treatment.

A power cut left parts of the hospital including the emergency department without power between 11pm last night and 3am this morning.

A Waitemata DHB spokesman said some areas of the hospital were without power after an issue occurred during scheduled maintenance works overnight.

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.

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.

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.