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.