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The fires erupted within 90 minutes on Saturday night and fire appliances from across the city responding, Senior Station Officer Lindsay Rae, of the Dunedin Central station, said.
The first, at the Green Island landfill, began about 9pm on Saturday inside the Beta Antifreeze building next to the landfill and resource recovery centre.
The second occurred at 10.20pm inside the clothing and textile room at Kavanagh College, extensively damaging the classroom and destroying sewing machines and other equipment, all of which was insured, school principal Tracey O'Brien said.
It appeared industrial dehumidifiers being used to dry the room after flooding from a burst water pipe may have caused the fire, he said.
Mr Rae said four fire appliances, a mobile command unit and about 20 firefighters were already ''stretched'' at the Green Island landfill when the Kavanagh College fire began.
Fortunately, members of the Ravensbourne volunteer brigade had been moved to the central city station as a contingency, in case there were any further callouts in the central city while so many resources were being used at the Green Island landfill fire, he said.
The volunteers were then sent to the Kavanagh College fire when the call came in, and were credited by Mr O'Brien with helping prevent the fire from spreading to other classrooms.
Mr Rae said it was a busy few hours for the city's firefighters, with units from Cental Dunedin joined by others from around the city at the landfill.
Firefighters used breathing equipment while fighting the fire, and a Fire Service spokesman said some were put through decontamination procedures as a precaution afterwards.
Mr Rae said it was initially not known what was inside the antifreeze recycling building, meaning it was treated as a hazardous materials callout, but nothing ''horrific'' was found inside.
The building's equipment was used to boil used antifreeze to separate it from the water it contained, he said.
The exact cause was yet to be confirmed, but fire investigators were ''looking pretty hard at the separating plant, because there's heat involved'', he said.
Dunedin City Council solid waste manager Ian Featherston said the council leased the site to Beta Antifreeze, but smoke detectors inside the building had worked and the landfill's operation was not affected.
Mr O'Brien said damage to the college's classroom and equipment would come at a ''fairly significant'' cost, but the school was fully insured.
He credited firefighters with stopping the fire from spreading, restricting it to one classroom and with only ''minor'' smoke damage beyond that.
He hoped the room could be refurbished in time for the start of the next school year, but that would depend on the availability of contractors and insurance matters.