State Highway 1 has reopened after a serious crash this morning that saw one person critically injured in Tokoroa.

The highway was closed for almost seven hours while emergency services investigated.

The two-vehicle crash happened about 6.05am between East Parkdale St and Main Rd, a police spokeswoman said.

A St John spokesman said one person had been flown with critical injuries to Waikato Hospital.

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Two ambulances and a helicopter went to the scene.

The road was closed and diversions were in place for a time.

A New Zealand Transport Agency tweet said the road reopened about 12.35pm.