Major delays: Auckland's Northern motorway re-opens after car flips

Traffic is again building on the Northwestern Motorway after a westbound bus broke down in Grafton Gully.

Motorists have been warned to expect delays as the crash is partially blocking a left lane on the link to the Southern Motorway, with congestion slow from the Port to Lincoln Rd.

A crash further up State Highway 16 has also closed the road between Old Railway Rd and Old North Rd.

These incidents come after one person was taken to North Shore Hospital after a car rolled on the Northern Motorway this afternoon.

Emergency services were called to the crash just north of the Onewa Rd off-ramp at 1.09pm.

A St John spokesperson said one person was serious and has been transported to North Shore Hospital.

"Two others were in a minor condition."

New Zealand Transport Agency said all lanes were reopened by 2.50pm but significant delays are still expected as congestion from the earlier crash clears.

"There is significant congestion with traffic backed up on SH1," NZTA said,

"An alternative way north is to go via the Western Ring Route on SH16 and SH18, but there is also congestion on SH16 from the Waterview Tunnel to Lincoln Rd.

"The mobile barrier on the Auckland Harbour Bridge is being moved early to provide five northbound lanes and improve traffic flows."

Traffic remains heavy between the Harbour Bridge and Upper Harbour Highway for northbound motorists on the Northern Motorway. Meanwhile, southbound congestion levels are increasing towards the Harbour Bridge.

Police are continuing to urge motorists to avoid the area if possible.

Congestion on the Southern Motorway is moderate and building at Manukau, citybound traffic is slow from Ellerslie to the Northern Link.

The Southwestern Motorway is heavy at the Southern Link for southbound traffic, meanwhile, northbound traffic is flowing well.

Northbound traffic on the Northern Motorway was being diverted off the motorway at Onewa Road and traffic had built up back across the Auckland Harbour Bridge.

Earlier this afternoon there was a tail queue at Newmarket as well as queues southbound near the accident due to rubberneckers, according to Newstalk ZB's Time Saver Traffic.

The Serious Crash Unit is attending the incident.