Two others who were travelling in a southbound car - including the driver - were taken to the same hospital via ambulance, suffering moderate injuries.
Two other people were treated at the scene, the spokeswoman said.
The road was closed between the railway overbridge and Paekakariki Hill Rd.
It was reopened shortly before 2.30pm, and police warned there would be delays while traffic clears.
The Labour Weekend road toll stands at one, following a collision on Auckland's Southern Motorway near Pokeno early this morning.
The official holiday road period runs from 4pm on Friday to 6am on Tuesday. Last year the Labour Weekend road toll was five, from 94 reported injury crashes.