State Highway 58, also known as Paremata Road, is blocked in both directions near Porirua after a two-vehicle crash reported about 5.05pm.
Police said two people were injured.
One person is in a serious condition and another has moderate injuries.
The Serious Crash Unit has been notified.
Motorists are being urged to avoid the area while emergency services work at the scene. Updates are expected through Waka Kotahi NZTA and Police channels.