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Witnesses say he appeared to fall back in the sulky while the horse continued to race, then fall onto the track.
Paramedics, racegoers and fellow horse people rushed to the Methven-based driver's side after the incident.
An announcement was made to the crowd of approximately 5000 racegoers that May was in a “stable, but very critical” condition.
A helicopter arrived at Omakau racecourse at 4.50pm to transport the leading driver to Dunedin Hospital.
A hospital spokeswoman on Thursday night confirmed that May was in a serious condition.
The Omakau races were called off at 5.10pm following the incident.
May is one of the most popular and successful harness racing drivers in New Zealand.
The driver has been a race track star, winning 2948 races in this country during a career of more than 40 years including the New Zealand Trotting Cup seven times.