He confirmed the incident happened just after the Te Irirangi Drive off-ramp and resulted in one person being trapped inside one car.
"They had to be extricated. And one car was upside down," the spokesman said.
Police confirmed one person suffered moderate to serious injuries in the incident.
Waka Kotahi NZ Transport Agency alerted motorists of the situation at 4.18am, saying on its website that a serious incident had occurred between that stretch of the motorway.
"Please delay your journey or use an alternative route."
Overhead motorway signs on the Northwestern Motorway also warned motorists of a serious incident near Te Irirangi Drive just after 4.30am.
The road has since been reopened.