"The man failed to stop for police about 6.30pm on Saturday. A pursuit was then initiated and the vehicle came to a stop near Cromwell," Sweeney said.
"The driver of that vehicle presented a firearm at an officer, before taking a police vehicle and fleeing the scene.
"He then travelled towards Alexandra, abandoning the police vehicle and taking a vehicle from a member of the public."
The Otago Daily Times understands no firearm was used in the carjacking.
Sweeney said police from Dunedin and Queenstown responded with two helicopters and AOS support.
"The helicopters tracked him a fair bit of the way."
The vehicle the man took was recovered in Milton about 11.45pm, after he fled on foot, Sweeney said.
No police issue firearms or equipment was outstanding, despite the stolen police car carrying firearms, Sweeney said.
The officer initially involved had been offered support after the event, and an investigation team had been established, he said.
As the incident was in its early stages, nine grass fires broke out along about 1km of State Highway 8 between Tarras and Cromwell and eight fire appliances were used to contain them.
Firefighters left the scene about 10.30pm.
Sweeney said the fires were "currently not linked" and he did not believe there was any damage to the police vehicle the man took that might have sparked the blazes.
"Not that I've heard," Sweeney said.
The arrested man was not known for lighting fires, he said.