One person is in critical condition after a car rolled on the Waikato Expressway.

Police warn there is likely to be some disruption to traffic following the crash this morning.

"A single-vehicle heading southbound rolled off the road onto railway tracks around 4:15am south of Ohinewai," says a police spokesperson.

"The vehicle has been removed from the tracks, and a man has been taken to hospital in critical condition.

"A crash scene examination will shortly take place and this is likely to affect traffic, particularly southbound.

"Motorists are asked to delay travel if possible and expect delays."