A woman has been critically injured when her vehicle left the road and ended up in a Counties Manukau paddock early today.

The incident happened on Buckland Rd near Tuakau just before 3am, police said.

A northern ambulance communications spokesman said the woman, in her 20s, drove through "a couple of fences" before ending up in the paddock.

She was taken in a critical condition to Middlemore Hospital.

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Buckland Road was closed just south of Tuakau Road while the Serious Crash Unit examined the scene.

A diversion was available via Buckville and Harrisville Roads.