A man is in a serious but stable condition after the vehicle he was in crashed on the outskirts of Ngaruawahia this morning.

However, exactly what caused the man's 2007 Honda people-mover vehicle to roll multiple times off the road is a mystery, say police.

Senior Sergeant Stephen Ambler, of Waikato Road Policing, said they're now calling for witnesses to the crash or anyone who saw the man's vehicle on Waingaro Rd or in the vicinity of Ngaruawahia during the early hours of this morning.

"It's rolled but we can't tell why or how. It seems to be the only vehicle involved and there only appears to be one occupant."

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Mr Ambler said the crash occurred at the Ngaruawahia end of Waingaro Rd and St John paramedics were called to the scene at 5.55am. Police weren't alerted until 7am.

However, he believed the crash may have occurred at any time during the early hours of this morning.

"There's an indication that the crash happened prior to [St John] responding to it, so it might have happened at any stage during the night."

The man was taken to Waikato Hospital in a critical condition but he had since stabilised.

Their investigation is hampered as ambulance staff who attended the crash were now off duty. The Waikato police serious crash unit are investigating in case the man's condition deteriorates.

Meanwhile, Mr Ambler confirmed the man who died in a slow speed crash in Kihikihi on Wednesday suffered a heart attack while behind the wheel.

Although they're still investigating when the heart attack occurred, Mr Ambler said it's likely it happened prior to 55-year-old David Spence crashing into a bank on Te Mawhai Rd, near State Highway 3.

Mr Spence suffered injuries after being propelled into the windscreen as he was not wearing a seatbelt.