The veteran hoop was aboard four-year-old mare Cerys in heat 18 of the day.
"The incident occured leaving the birdcage and the horse reared up and ended up rearing over, landing on Michael's leg," Stipendary Steward Brady Jones said.
"He was attended to immediately by paramedics and the injury is defined as a leg injury until x-rays are taken.
"It wasn't anything out of the ordinary. It was just unfortunate how it all happened. The horse just lost its balance and Michael ended up half underneath it."
The popular rider is one of New Zealand's most successful ever jockeys, with more than 2100 domestic wins to his name.