Leading New Zealand harness racing driver Ricky May is reportedly in a critical condition after suffering an apparent heart attack during a race at the South Island track of Omakau.

May fell back into the sulky then on to the track when he was leading Thursday's race.

Medical staff worked on May for some time to stabilise him so he could be flown to Dunedin Hospital.

Stipendiary steward Vinny Munro said May had suffered a "medical event", believed to be a cardiac arrest, stuff.co reported.

Munro praised the work of the people who rushed to May's aid, including two off-duty doctors.

The meeting was abandoned after drivers and trainers were consulted.

"Due to the condition of those people, health and safety is paramount, and we have called the meeting off," Munro told Trackside.

May is one of New Zealand's most successful and experienced harness racing drivers and has won almost 3000 races during a career of more than 40 years.