Emergency services are at the scene of a serious crash on the Mackays to Peka Peka expressway, Otaihanga.
Police were called about 1.52pm.
A person involved in a single-vehicle crash is understood to have left the vehicle, and has been hit by another vehicle.
Initial reports suggest the woman is in a serious condition.
The road is currently down to one lane and may be closed for a short time.
Motorists are advised there will be delays and should avoid the area if possible.
The Serious Crash Unit will examine the scene.