A crash on Auckland's Southern Motorway is adding to rush-hour congestion in the country's biggest city.

The New Zealand Transport Agency said the crash was blocking the citybound right lane before the Mt Wellington off-ramp.

The agency said motorists should expect delays and take care.

Meanwhile, trains are now running with a 20 minute frequency between Britomart and Papakura after a freight train breakdown earlier caused delays.

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Auckland Transport said the train had broken down at Te Mahia in Manurewa.

"Unfortunately this will result in disruptions," it said.

Scheduled buses had earlier been accepting rail tickets.

A truck breakdown was causing delays north of Auckland.

The New Zealand Transport Agency said the truck had broken down in the southbound lane of State Highway 1 near Pohuehue, between Puhoi and Warkowrth.

The breakdown caused long delays to traffic in both directions until the truck was cleared.

However, the agency said motorists should still expect delays as congestion eased.