Two people suffered serious injuries when their car flipped in a crash that has closed all city-bound lanes on Auckland's Southern Motorway near Otara.

Two people have been taken to Middlemore Hospital. Their car flipped and ended up on its roof in the crash at 1.07pm. No other vehicles were involved.

A motorist who drove past the crash said emergency services staff had to cut the occupants out of the vehicle.

"It does look to be quite a serious accident, and the traffic is piling up. It looks pretty nasty."

The Southern Motorway is now closed northbound between State Highway 20 and East Tamaki Dr.

UPDATE 1:40PM
The Southern Mwy is NOW CLOSED northbound between SH20 and East Tamaki Dr. Please use SH20 via Waterview Tunnel to go north or detour to East Tamaki Dr for access to SH1 northbound. ^TPhttps://t.co/YROrjBAk0D

— NZTA Akld & Nthlnd (@NZTAAkl) November 25, 2019

Police said the northbound motorway lanes were expected to be closed for some time, and warned of "significant delays".

All four lanes of the motorway northbound are currently closed between Redoubt Road and East Tamaki Road, police said.

Motorists were also unable to travel on the Te Irirangi Drive on ramp, and northbound traffic was being turned off at Redoubt Road.

Great South Road is also expected to be congested.

Motorists are advised to use SH20 via the Waterview Tunnel to go north, or to detour to East Tamaki Dr for access to SH1 northbound, NZTA said.

One southbound lane is also blocked under the Reagan Rd overbridge.

Motorists are urged to avoid the area as there are now delays in both directions.