"The member of the public was shaken, but unharmed," Sweeney said.
"The police vehicle was recovered."
The man then travelled south along State Highway 90.
The vehicle has been recovered in Milton about 11.45pm, but the driver is still missing.
"Police are actively searching for the man, with the Armed Offenders Squad in attendance," Sweeney said.
"We would urge the public to be alert, but not alarmed, and to report any suspicious activity in the area to Police on 111."
Further information was to be provided this morning.
Meanwhile, there have been several fires in the area and a vehicle is believed to have crashed into the river at Beaumont.
It has not been confirmed whether the fires or the crashed vehicle are linked to the offender.
Some of the AOS deployed are from Queenstown and it is understood helicopters are part of the operation.
Witness Emily Cannan said she saw a silver car speeding past Tarras about 7pm with police hot in pursuit.
The area near Cromwell was teeming with police and firefighters and there were several fires on the hillside.
There were up to 40 firefighters in the area and multiple police cars.
Another witness said there were many emergency services vehicles down by the river at Beaumont, including multiple fire appliances and a St John ambulance.
She said it appeared a car had crashed into the water.
A St John spokeswoman said they called to the scene at 8.11pm and were still at the scene at 9.06pm.
It was unclear if the crash was linked to the offender.
Commenters on social media reported seeing police cars travelling at speed in the Millers Flat and Lawrence areas.
