Crews had the fire under control and were dampening down hotspots just after 7.15am.
Fire investigator Jason Hobbs said the unit was totally destroyed in the blaze but two adjoining flats were saved as they had fire walls between them.
The fire is thought to have been deliberately lit but the circumstances of the blaze were still being determined on Tuesday afternoon.
The unit involved is owned by the Christchurch City Council.
Joseph Johnson/Stuff
Fire crews at the scene of the blaze, which is believed to be suspicious.
Ōtautahi Community Housing Trust (ŌCHT) tenancy and property general manager Robert Hardie said trust staff had been to the scene to undertake “welfare checks” on those living in the unit and the two adjoined flats.