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The fire erupted in early yesterday, on a steep hillside above Kaikorai Valley College, near Marshall St in Bradford.
Residents evacuated several nearby homes before the blaze was brought under control by about 4.30am.
Crews of professional firefighters from St Kilda, Lookout Pt, Roslyn and Dunedin Central stations battled the blaze as it consumed about half an acre of scrub and gorse.
Fire and Emergency NZ fire risk management officer Mike Cahill, of Invercargill, was at the scene investigating yesterday, as this week's fire investigator on duty in Otago and Southland.
"I believe it's likely to be suspicious.''
Mr Cahill said the fire had burned fairly close to several homes before it was contained.
The residents of those homes had evacuated the properties for a short time as the fire ravaged the hillside.
Senior Sergeant Craig Dinnissen, of Dunedin, said the Criminal Investigation Branch was now involved in the case.