Two people are recovering in hospital tonight after being involved in separate car crashes north of Auckland this afternoon.

The accidents, which occurred within about 15 minutes of each other after 2pm, resulted in significant delays on the northern motorway this afternoon.

Heavy delays in the area continued as peak-hour traffic started.

All lanes affected have since been cleared and opened early this evening. However, authorities are telling motorists to expect some delays still, as congestion eases.

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One of the crashes, which happened just before 2.30pm, resulted in the serious injury of a person who was taken to North Shore Hospital for treatment.

The accident happened near Bawden Rd, in Redvale, and motorists were being urged to use the Dairy Flat Highway to Albany route to avoid the area altogether or to delay travel.

A second person was critically injured in a different crash, also after 2pm, on the State Highway 1 Northern Motorway between Silverdale and Oteha Valley Rd.

St John said the victim was in a critical condition at the time of transport and was taken to Auckland City Hospital.

Just after 5pm, the NZ Transport Agency reported traffic on the Northern Motorway, northbound, as being heavy between the Harbour Bridge and Northcote Rd.

It was also heavy from Oteha Valley to Redvale, where one of the crashes occurred.

Citybound on the northern motorway, the lanes had been cleared just before Redvale, but authorities warned of "significant delays'' from Silverdale approaching the Harbour Bridge.

On the Southern Motorway, there is heavy congestion southbound approaching Greenlane and again at Penrose Rd.

Traffic is also heavy from Manukau to Takanini and citybound, congestion is heavy between Princes St and Greenlane.

Those heading out west, on the Northwestern Motorway, can expect heavy traffic from Great North Rd to Lincoln Rd, Henderson.

It is also slow approaching the Brigham Creek roundabout. Citybound, congestion is also heavy approaching the Northern Link, while traffic heading into the Waterview Tunnel (southbound) can expect free-flowing traffic.

Meanwhile, on the Southwestern Motorway, there are heavy delays between George Bolt Memorial Dr and Neilson St on the northbound lane.

Southbound, traffic is heavy approaching the Southern Link and there are no delays in the Waterview Tunnel heading north.