Motorists were urged to follow directions by emergency services and to expect significant delays in both directions as the queues cleared.
Waka Kotahi said at 3.15pm that southbound traffic was backed up to Warkworth while northbound traffic was queuing from Orewa to get through.
Further north, State Highway 1 was closed about 4pm between Kaitaia and Cape Reinga due to smoke from an ongoing bush fire in the area.
The Kaimaumau fire started on December 18 and by Boxing Day had swept through 2360ha of scrub and wetland, most of it conservation land.
Residents had to evacuate their homes for a second time yesterday afternoon and people were warned to stay away from beaches in the area as firefighters closed Kaimaumau Rd.
Waikato
In Waikato, State Highway 25 has now reopened at the Hikuai River Bridge after earlier being closed due to a crash but significant delays are still expected.
Congestion warnings were earlier put in place on SH25 for both directions at Tairua and southbound on SH1 between Cambridge and Tirau. The agency urged motorists to allow extra time for their journeys and to expect delays.
A motorist posted on the Thames/Coromandel Newsfeed & Grapevine Facebook group saying the jam was at least 5km long into Tairua from Whitianga.
Road toll
The holiday toll reached 12 today, surpassing last year, after a person died in the Waikato after a crash that brought down power lines.
Emergency services were called to the site of the crash on Arapuni Rd between Putaruru and Te Awamutu about 2.40pm.
The person was trapped in the vehicle and a small fire started next to the car.
Police confirmed a person had died following the single-vehicle crash, and warned travellers to avoid the area.
"The road remains blocked in both directions as power lines were brought down, and it is likely to be closed to traffic for some time."
Earlier, a motorcyclist died after coming off the road in Northland just after midnight.
The man died at the scene on State Highway 12 in Kaihu around 12.20am.
A motorcycle passenger received moderate injuries and was taken to Whangārei Hospital for treatment.
The Serious Crash Unit is investigating.
The Christmas-New Year holiday road toll period finishes at 6am on Wednesday. Eleven people died during last year's holiday period.