"What happened is there was an electrical fire in a circuit switchboard. That triggered the fire alarm and the sprinkler system," Dr Sage said.
The hospital's emergency department and the acute planning unit were evacuated.
"What most people would have noticed ... was the smell of the smoke. That wasn't pleasant. It [the fire] was put out; smoke was the issue. Electrical [fire] smells are pretty, sort of, acidic. It's not a comfortable smell," he said.
The ED was evacuated to the ambulance bay and patients still requiring attention were treated in the open.
"The emergency department has 28 bed spaces and it was busy at the time, so there would have been over 20 or maybe 30 [patients]. About 60 people would have left that part of the hospital.
"Luckily we weren't involved in any resuscitation. None of their treatment was jeopardised. I know it sounds silly but it was business as usual, but outdoors," he said.
Patients requiring more serious treatment or needing privacy were treated in parked ambulances.
Other patients lay in the sun with sheets above to provide shelter and ice blocks were handed out.
St John brought their mass casualty trailer which includes a mobile hospital - a tent which could have been erected if required.
The fire had disrupted power to certain parts of the hospital. The emergency department had to be moved to the medical day-stay unit and downsized - by about 33 per cent - until the hospital returned to full operation.
Tauranga station Chief Fire Officer Nigel Liddicoat said firecrews had initially had trouble finding the source of the fire as smoke circulated through the hospital's air-conditioning, setting off a number of alarms.
A Greerton fire unit passing the hospital was alerted to black smoke coming from a basement air-conditioning grate, which helped fire crews target the origin of the blaze.
A combination of hospital sprinkler systems and firefighters extinguished the fire.
Jenny Turner told the Bay of Plenty Times she had just arrived at the hospital complex to visit a patient when she smelled the fire.
"You can smell like an electrical burning smell. I thought it was the car [at first]. It was quite strong," she said.
Amer Basham, 11, had been riding her bike through the hospital grounds.
"We saw heaps and heaps of smoke and people assembling at the front of the hospital. There was lots of black smoke coming out of there," she said, pointing at a caged door to the basement.
Jules Rielly, also visiting at the hospital, said there had been smoke coming from the building when she arrived, but praised the calmness of the evacuation. "I saw a bit of smoke coming out. But I would say it all looks pretty calm and under control," she said.
ED relocate back to the Emergency Department at 6am today.
