Hokitika Volunteer Fire Brigade chief fire officer Harry Collett said the split-level building was already ablaze when the brigade arrived about 1am after multiple calls from neighbours.
Back-up was called from the Ross, Kumara and Greymouth brigades, which spent about four hours at the scene.
Mr Collett said they were told the last tenant in the flats had moved out on Sunday, while the occupant of the adjoining two-storey manager's house was safely evacuated.
The fire appeared to have started in the middle unit, while several separate units to the rear of the complex had not been torched but were vandalised, he said.
"We are treating it as suspicious at this stage, until an investigation has been completed."
The site has been on council's radar in recent months following a complaint from Hokitika police about health and safety after rubbish began piling up at the main road property. Police described the flats as a "breeding ground for crime".
During a visit to the property in September, Mr Guyer told the Guardian three people were living there at the time but others would stay short-term or overnight.
In the Greymouth District Court last week, Mr Guyer was bailed to his mother's house in Nelson when he appeared on charges of burglary, as well as possessing an imitation firearm and disorderly behaviour.
His lawyer Tony Bamford explained to the court there was a "back story" around the offending, which arose after a group of men turned up at the flats, smashed the windows and tried to set fire to the unit.
The attack happened after a previous partner of Mr Guyer, with whom he had finished a relationship, had got involved with someone else.
Mr Bamford claimed in court the new person had tried to dole out retribution, which included death threats on Facebook.
That had left Mr Guyer petrified, and he had fled to "cower" in a nearby garage, where he had armed himself with a screwdriver and other tools, Mr Bamford said.
Judge David Saunders said there was "real concern about what was happening in Hokitika" and expressed concern someone was living in a former motel complex which was unfit for habitation.
- Hokitika Guardian