The closure point on the Canterbury side is half a kilometre east of Castle Hill Village, between the village and Thomas River Bridge.
Electronic signs are advertising the closure.
Drivers can take the Lewis Pass route (SH7) via Waipara as an alternative slightly longer but fully open route.
The road was initially closed for several hours on Wednesday.
"Fortunately, the driver appears to have been uninjured," a police spokesperson said.
No ambulances were called to the scene.
By 4.55pm, the road had partially reopened and at 7.15pm, Waka Kotahi said State Highway 73 was open with stop/go signals in operation.
