One person has died and another four have been injured in a crash on State Highway 1 in the north of Auckland.
The crash - which happened near Cowan Bay Rd in Warkworth - caused the closure of a section of SH1 between Puhoi and Warkworth, leading to extensive traffic delays.
NZTA tweeted that the section of SH1 "is now open" at 6.55pm.
One nearby resident said the traffic was the worst he'd ever seen in the eight years he had lived in the area.
Grant McLoughlin called it "absolute madness".
St John attended the incident at 1.38pm. Initially four people suffered moderate injuries and one had critical or life-threatening injuries.
Police later confirmed the critically injured person had died. One of the people listed initially as moderately injured is now in a serious condition.
Drivers have been diverted off SH1 and through to Makarau and SH16. It is estimated to take 50 minutes to get from Puhoi to Warkworth, as of 6pm.
Since then, another two crashes occurred within half an hour.
At 4.40pm, one car crashed on SH16 just south of Wellsford, leaving one person with minor injuries.
Shortly before 5pm, a campervan smashed into a fence on Woodcock Rd between Old Kaipara Rd and Edgerley Rd. No one was injured but a police spokeswoman said they received complaints of traffic delays.
McLoughlin said he was hearing of other crashes around the area as traffic built up extensively.
AA principal adviser advocate Martin Glynn said the project between Puhoi and Warkworth would help ease the pressure on SH1 and this crash highlighted why it was so important.
"A crash like this is tragic, the road has to be closed because police need time to investigate, but it just shows why we need more reliance ... the network should be able to cope so one crash doesn't shut things down like we have seen today."
Glynn said the whole stretch, including north of Warkworth, was dangerous and given the volumes of traffic, dual lanes needed to be looked at.
People stuck in traffic have taken to Facebook, one saying the stretch of road was "so dangerous with impatient drivers".
Another said: "It's gridlock in all directions ... Flat Rd is the end of the queue going north via SH16."
Five St John vehicles attended the crash on SH1 and a helicopter was also dispatched.
Just before 3pm, Waka Kotahi NZ Transport tweeted that the state highway was closed between Grand Drive in Orewa and Woodcocks Rd in Warkworth.
"[It] is likely to remain closed for some time today to allow for a serious crash investigation. Delay your journey or use SH16 via Kaukapakapa as an alternative route," NZTA said.
Police said the explosive - believed to be an old grenade - has been taken away.