Traffic is backing up on the Southern Motorway as Aucklanders leave the city for the long weekend.

The motorway is heavily congested southbound at Manukau and queues are building at the State Highway 1-State Highway 20 interchange.

NZTA says SH1 is expected to be busy between Manukau and Papakura until 1pm.

Road users in the area should expect delays and allow extra time, the agency said.

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Meanwhile, SH2 is closed north of Katikati after a serious crash that has left one person fighting for their life and to others injured.

Police were called to the incident, which involved two cars, at the intersection of the highway and Kauri Point Rd near Tahawai at 8.44am.

Senior Sergeant Glenn Saunders said one person had been critically hurt in the crash and two others had moderate injuries, and all were being transferred to hospital.

"The Serious Crash Unit is on its way to the scene and as a result, the road has been closed in both directions and is likely to remain shut for several hours," Saunders said.

There are no diversions in place so police ask motorists avoid the area and thank people in advance for their patience.

Roads are also expected to be busy north of Auckland.

NZTA said traffic on SH1 northbound between Puhoi and Warkworth was likely to be heaviest between 10am and 2.30pm.

Signs above the northbound lanes on the Harbour Bridge are warning motorists to expect delays between Orewa and the Far North.

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