A driver is trapped after losing control of the vehicle and hurtling over a 15-metre bank.
The motorist is believed to have sustained minor injuries in the crash, on State Highway 58, between Pauatahanui and the Hutt Valley, senior sergeant Marc Clausen said.
Emergency services are at the scene, and the road has been partially closed while they try to free the driver.
Firefighters have begun cutting the vehicle open to get the driver out.
It was not clear whether any other vehicles were involved in the incident, but only one had gone over the bank, police said.
It appeared the driver had lost control of the vehicle.
Porirua to Hutt Valley traffic is being diverted via SH1. The Hutt to Porirua lane is open but extreme care is required and delays should be expected, Mr Clausen said.