Police believe the crash happened between 11.40am and 12.05pm.
Officers are working to identify and locate the vehicle and its driver.
Police want to hear from anyone who travelled along Pākōwhai Rd, Evenden Rd or the expressway around that time, witnessed the crash or has dashcam footage.
Information can be provided online at 105.police.govt.nz by selecting “Update Report” or by calling 105 and quoting event number P067903799.
Information can also be provided anonymously through Crime Stoppers on 0800 555 111 or at crimestoppers-nz.org.