A motorist who suffered a serious medical event during morning rush hour led to traffic chaos on Auckland's Southern Motorway this morning.

The incident just before 7am led to traffic being gridlocked back to Manukau, with motorists warned to stay off the motorway if they are heading into the city this morning.

Police say the incident happened in the fast lane just before the Ellerslie-Panmure off-ramp heading north. There was also a minor three-car crash in the same area.

The crash was cleared from the lanes just after 8am though traffic remained banked up for 17 kilometres. The delays had mainly cleared by 9am, the NZ Transport Agency said.

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Police has earlier issued an urgent plea for motorists to delay any unnecessary travel on the congested highway and take alternative routes. 

A driver was in a critical condition and being treated by St John Ambulance staff at the scene.

Police said the northbound lanes were blocked and significant delays were expected while emergency services dealt with the incident.

Motorists were asked to exercise patience with no time frame for the delays at this stage.