A second person has been confirmed seriously injured in the blaze that lit up the Whanganui skyline last night.
Two people were rushed to hospital - one with life-threatening injuries - following the fire at the corner of Victoria Ave and Taupo Quay in the city centre.
More than a dozen fire crews from as far away as Palmerston North rushed to the fire after multiple calls came in from 8.45pm.
Last night it was reported that one person was critically injured, with fire crews only able to get into the building at 10.30pm. At that stage it was unknown if anyone else was trapped in the building.
"We are unsure if people are still inside and if others have been injured at this stage," Fire and Emergency New Zealand shift manager Carlos Dempsey said on Saturday night.
Police and the fire service confirmed this morning that a second person had been seriously injured. Both those caught in the fire had been treated for burns and smoke inhalation and taken to Whanganui Hospital, a police spokeswoman said.
At the height of the fire more than 100 people gathered outside in shock, with a eye-witnessed describing it as a "massive blaze".
"I was driving home and I just looked up and I could just see huge flames, like crazy big flames, it was pretty scary," witness Maddy Stoneman-Boyle told the Herald.
She said smoke was billowing more than 50 metres above the burning building.
"The road is blocked off so I'm watching from a distance, a lot of people standing outside in shock, like easily more than 100 people," Stoneman-Boyle said.
Police had blocked off the road.
Photos posted on social media showed smoke billowing from a building at the bottom of Victoria Ave, near the river.
A witness told the Herald several nearby restaurants had been evacuated.
Dozens of comments of people in shock were being posted in response to photos of the blaze.
The fire was at heritage Thain's building at 1 Victoria Ave which was sold after an earlier consent to demolish it had been declined.
The Thain's building is a Class B heritage building in the Whanganui District Plan but has no national heritage status.
The three-storey unreinforced masonry building is severely earthquake-prone at 5 per cent of new building standard (NBS) and the ground floor was flooded in the June 2015 floods.