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Auckland's Northern Motorway southbound is open again and traffic is flowing at normal levels after a truck crash forced its closure mid-morning.

Transport authorities believe the peak-flow this evening will not be any busier than usual.

The overbridge will be demolished overnight and the motorway will be closed until about noon tomorrow.

Southbound diversions will be in place but police are appealing to motorists to stay put unless travel is absolutely necessary.

Earlier police told trapped drivers to turn off their engines after a truck carrying a digger on its tray crashed into the old Onewa motorway ramp, raining concrete on cars and blocking traffic.

Waitemata area commander Les Paterson had expressed safety concerns about the crash site.

Land Transport Agency acting operations manager Mark Walker said the preparation work on the demolition of the bridge has already been done.

Motorist Richard Lissington said he was only a matter of metres away from a car that got hit by raining concrete and other debris.

He said a piece of concrete fell through the back window of a Volkswagen car and the woman driver was seen to by St John staff.

"I just heard this noise and looked up and it came down. All I saw was debris coming down," Mr Lissington said.

He said the woman was lucky.

"She should buy a lotto ticket. I am, I'm going to have a few beers tonight," Mr Lissington said.

Another motorist Andrew Redwood said traffic heading southbound could "forget about it, it's totally jammed up".

Mr Redwood called nzherald.co.nz from the motorway shortly after the accident to say that chunks of concrete and reinforcing steel rods were on the road.

"The digger went all the way through. There's steel and concrete all over the place," he said.

Mr Redwood said ambulances were at the scene and traffic was backing up after no-one was able to move for 15 minutes.

Earlier Inspector Kerry Watson said drivers caught on the motorway could do little, other than wait for the chaos to clear.

He said if they did not look like moving, they should turn off their engines and read whatever they could find in their car.

He also urged motorists not to back up the motorway looking for an off ramp.

"That could cause a few other issues. Sit and wait or take alternative routes."

A St John Ambulance spokeswoman said no one was transported to hospital from the scene.