Fire investigator Gary Beer said firefighters used a low pressure hose to bring the blaze under control and stopped the fire spreading to the other two flats.
The flat was empty at the time.
Mr Beer said the fire appeared to have started from the lounge.
It's not clear whether the occupants were owners or were renting the flat.
The fire followed a blaze that gutted a three-bedroom home in Raumanga, Whangarei, on Thursday last week.
Four fire trucks and about 20 firefighters attended the fire on Fairburn St, Raumanga, about 12.30pm.
A male occupant was taken to hospital with smoke inhalation.
Hours earlier the firefighters had delivered gifts to children in Otangarei as part of the Christmas shoebox project.
That blaze came just weeks after a Mamaranui house was devastated by flames, and nearly a month after a family of five were left with only the clothes on their backs after their rental property was destroyed in a fire in November.