All lanes have reopened after a crash on the Waikato Expressway near Mercer.

Emergency services responded to a single-vehicle crash on State Highway 1 near Mercer, reported to police just before 2pm.

One person has serious injuries, while three have moderate to minor injuries.

A Hato Hone St John spokesperson said a patient in a serious condition and two in moderate conditions were taken to Middlemore Hospital.

A Fire and Emergency New Zealand spokesperson said crews assisted Hato Hone St John and police.

The Serious Crash Unit has been notified.