State Highway 1 is blocked and six people injured in a crash in South Waikato.

One of the injured remains trapped in one of the two vehicles involved in the crash which happened just after 1pm today.

Police are currently putting diversions in place between White and Webster Rds.

A St John spokeswoman confirmed two people were in a critical condition and four others have suffered moderate injuries.

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Northern fire communications shift manager Karen Larking said firefighters have arranged a landing pad for a rescue helicopter to transport the injured to hospital.

There were two fire appliances and one support vehicle at the scene.

As well as the rescue chopper, St John have three ambulances and a rapid response vehicle helping the injured.