A truck's dangerous cargo is being carefully transferred from an upturned trailer north of Auckland.

The truck, carrying dangerous chemicals used in explosives, crashed about 9am, blocking a route used by traffic to miss an earlier major truck crash.

Police say the alternative route around eastern Northland coastal roads to bypass the Brynderwyn crash is impassable after a second truck went off the road.

A section of State Highway 1 at Brynderwyn was blocked after a truck and trailer rolled about 4am today. It has now reopened.

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About two hours later a stock truck is believed to have gone off road while taking the Mountain Rd diversion. A tow truck which was on its way to another crash pulled the truck out.

Then a large truck and trailer carrying hazardous material crashed on Cove Rd, Mangawhai, the coastal highway being used as a diversion.

Police said that crash happened just after 9am between Woodleigh Lane and Bream Tail Rd.

Nothing was leaking from the truck but emergency services set up a safe space around the crash.

Fire Service northern communications shift manager Scott Osmond said the truck's dangerous cargo of ammonium nitrate was being carefully transferred.

The compound is predominantly used in agriculture as a high-nitrogen fertiliser. Its other major use is as a component of explosives.

Osmond said despite the crash the chemicals were still intact inside containers.

Four crews from Whangarei and Mangawhai were involved in the operation.

Meanwhile, SH1between Baldrock Rd and SH12 at Brynderwyn finally reopened to traffic after the first crash just before 1pm.

A crane was needed to move the overturned truck and trailer.

Traffic filing past the crash was earlier warned to expect long queues.

Police used stop/go signs to guide the building traffic through on a single passing lane.

It is not yet known when the coastal route will be reopened.