A motorcyclist is dead after a crash on Auckland's Southern Motorway this morning, and motorists are being warned to expect delays.

The motorway has now fully reopened after the fatal crash at Takanini.

The motorway was briefly closed at 8am before a single lane was reopened about 20 minutes later.

The New Zealand Transport Agency said at 9am all northbound lanes were open.

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However, traffic remained heavy and motorists should delay their travel, avoid the area or allow plenty of time for their journey.

The northbound lanes between Papakura and Takanini would be closed tonight when traffic flows were reduced so police could complete their investigations, the agency said.

The New Zealand Transport Agency's Auckland and Northland Twitter account posted just before 8am that an earlier crash on the Southwestern Motorway's northbound lanes, near Portage Rd, was now clear.

However, motorists should allow extra time for congestion to ease.

Both crashes happened just before 7.30am.