Motorists headed from Wellington to the Kāpiti Coast on State Highway 1 are being urged to delay their travel after a truck and two cars collided.
Police said there were reports of injuries following the crash, which happened in the northbound lane between Pukerua Bay and Paekakariki.
The NZ Transport Agency said the crash had been cleared, but traffic remained at a standstill in the area.
Newstalk ZB reporter Katrina Bennett was driving on Paekakariki Hill Rd, which she said was "really congested".
"Cars aren't moving quickly at all - it's heavy in both directions.
"It's just crazy. I can see State Highway 1 now and it's chocka."
She said the crash on SH1 looked very serious, and she could see the Westpac Rescue Helicopter heading in.
Southbound traffic was being diverted over the Paekakariki Hill Rd, however this is not suitable for trucks and heavy vehicles.
Northbound traffic was being diverted down the southbound lane of SH1.
Motorists should expect lengthy delays, police said in a statement.