"A salvage team has been sent to the scene to right the unit, which is half in a ditch and half on the road."
Police say traffic disruption is expected.
A Fire Service spokesman said emergency services at the scene were trying to disconnect the truck from the trailer unit, the Otago Daily Times reports.
The fuel would then be decanted from the trailer and the vehicle righted.
It was believed the road had reopened to one lane, but the work to right the truck and trailer was expected to take much of the day, the police spokeswoman told the ODT.
The Commercial Vehicle Investigation Unit has been informed and will investigate the cause of the crash.