Firefighters were able to extinguish the blaze but there was extensive damage to the property.
Whanganui Fire Station manager Jason Hamlin, who attended the blaze, said there were three flats in the building.
"We were able to contain the fire before it spread to the other flats," he said.
"The one that was on fire is extensively damaged but everyone got out okay."
One flat in a Wicksteed St property was extensively damaged before firefighters extinguished the blaze on Saturday.
Photo / Lewis GardnerHamlin was unsure whether there were working smoke alarms in the flats.
"It is timely to remind everyone to check their smoke alarm batteries because they do save lives," he said.
The cause of the fire is being investigated.