A truck driver who jack-rolled and rolled on the southbound off-ramp to Pōkeno shortly after 2pm was taken to hospital in a critical condition.

The driver, a man in his 40s, was still trapped in the rolled truck when the Auckland Westpac Rescue Helicopter arrived.

Crewman and co-pilot Aaron Knight said it's believed he lost control of the logging truck as he came downhill towards a round-a-bout.

As a result, the truck and trailer jack-knifed then rolled by the State Highway 2 off-ramp.

The driver suffered multiple injuries, mostly to his lower body, and was taken to hospital in a critical condition, a rescue helicopter spokesman said.

"It was elected to transport the patient by road to Middlemore Hospital as this was the faster option as opposed to taking off, landing, loading and unloading from the chopper," the spokesman said.

"The patient was accompanied by our Intensive Care Paramedic and HEMS (Helicopter Emergency Medical Service) Doctor in the back of the ambulance."

Earlier today, two separate crashes wreaked havoc on traffic travelling in both directions on State Highway 1, including the truck crash.

The first involved two trucks in the northbound lane of SH1 that has caused major delays, with traffic being diverted.

Ambulances and fire crews are also on the scene, and a rescue helicopter were sent to the scene.

Then, in a second crash, a truck has rolled at the SH2 southbound off-ramp to Pōkeno that was being used to divert traffic away from the first crash.

NZTA was advising motorists to delay their journeys until alternative detours are in place.