The slip, below the highway, occurred at approximately 4pm yesterday. One lane of the road is currently closed and stop/go traffic management is in place.
Geotechnical experts are assessing the site today.
The road is closed to trucks and other heavy vehicles, which could cause more movement at the site.
Heavy vehicles can use State Highway 25A as an alternative route.
A further update will be issued later today following the geotechnical inspection.
For real time information on highway conditions and incidents or to report issues on the network visit NZ Transport Agency's traffic and travel page, call 0800 4 HIGHWAYS (0800 44 44 49) or by following the Transport Agency on Facebook or @nztawaibop on twitter.
Motorists can also go to Drivelive, a dedicated website that tells people what the current travel times are to get to key places around the North Island.