A firefighter was one of two people injured following an explosion on Church Street in central Wellington.
Te Whatu Ora said one patient was in a stable condition in the emergency department, and the other was in a serious condition in ICU.
Emergency services were called to the Church Street address at 1pm on Friday afternoon.
Police, fire and ambulance were on the scene, and both Church Street and Boulcott Street were cordoned off to traffic.
Wellington Free Ambulance said two people were taken to Wellington Hospital, one person in a serious condition and one in a moderate condition.
Police and FENZ confirmed one of those was a firefighter.
The building, which is leased by Kainga Ora and contains up to 100 apartments, was evacuated.
Kainga Ora's regional director for Greater Wellington, Vicki McLaren, said they were "very concerned" to learn about reports of the explosion.
"Our immediate priority is checking on the safety and wellbeing of our customers and we have staff onsite who are ready to offer support once we can access the building or reach the customers by phone."
FENZ assistant commander James Martin-Bond said they received a call from an alarm activation.
"Upon arrival we had reports of a fire on level eight. When crews investigated, we found there had been an explosion in one of the apartments."
He said eleven crews responded, the whole building was evacuated and they were now investigating the cause of the explosion.
Boulcott Street reopened after 5pm.