Police have advised motorists to avoid the Haast Pass Highway, as the clean-up was expected to take most of Sunday.
A police spokeswoman confirmed the driver of the tanker was not hurt and no other vehicles were involved when the truck tipped over at 7.53am.
The Wanaka-bound tanker rolled and was blocking State Highway 6 between Haast Pass-Makaroa Rd and Marks Rd.
A salvage team had been sent to the scene to right the unit, which was half in a ditch and half on the road.
A Fire Service spokesman said emergency services at the scene were attempting to disconnect the truck from the trailer unit.
The fuel would then be decanted from the trailer and the vehicle would be 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 said.
"Avoid the area if possible and delays are expected."
The Commercial Vehicle Investigation Unit had been informed.
