Waikato police are seeking witnesses of a "serious" crash that occurred on State Highway 39 south west of Hamilton on Thursday afternoon.

A motorcycle and car collided near Rice and Allcock Roads just after 2pm.

The rider is in critical condition in Waikato Hospital and the driver of the vehicle is uninjured.

A Waikato man, who was fourth on the scene, said the motorcyclist was travelling south and was riding a registered dirt bike.

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Police are seeking witnesses, particularly the driver of a small silver hatchback.

"We believe the vehicle was following directly behind the motorcycle before it crashed and that the driver was a man wearing an orange hi-vis vest," senior constable Mark Strongman said.

"Additionally, we are looking for the occupants of a vehicle belonging to an insulation installation company that was also in the area at the time."

"These people are being sought solely as witnesses and could greatly help our investigation."

Anyone who witnessed the crash should contact sergeant Warren Shaw or senior constable Mark Strongman on 07 872 0100.