A Fenz spokesperson confirmed one of their support vehicles rolled while travelling to an incident in Fairlie at 5.20am.
The single occupant was initially trapped in the vehicle, they said.
"Fire and Emergency crews from Fairlie and Geraldine attended the incident, and the occupant was successfully extricated. They were then transported to hospital by Hato Hone St John."
A Hato Hone St John spokesperson said St John responded with one ambulance and assessed, treated, and transported one patient to Timaru Hospital in a serious condition.
The accident occurred as crews battled a fire at a block of shops in Fairlie.
Crews from Fairlie, Pleasant Point, Geraldine, Timaru and Washdykes were sent to extinguish the blaze, which was reported on Main St around 2.10am.
The building was “well alight" when the first two fire trucks from Fairlie arrived and they quickly asked for extra resources, Fire and Emergency’s southern shift manager Lyn Crosson said.
The fire was contained around 3.30am but the crews were still working to totally extinguish the fire.
Geraldine chief fire officer Graeme Mould told NZME two shops had been damaged in the fire.
He said the fire had started at the fish and chip shop but spread to the building next door, which he believed was empty.
"Both of them are gutted ... It was quite severe," he said.
The smoke had filled the bakehouse initially but only the smell of smoke remained.
Fire crews were still on the scene just before 7.30am.
Waka Kotahi NZTA said State Highway 8 was closed near the intersection of Talbot Rd.
As of 6am, there were still four fire engines and a tanker in attendance - however, a crew from Fairlie was called away to attend the FENZ support vehicle crash, Crosson said.
“When we first arrived, there was one person still in the vehicle," she said.
Police, ambulance and roading contractors had been advised.
Motorists were advised to be vigilant as the crash was on a corner on a windy road.
