A "significant hit on her head" left Channel Nine presenter and journalist Tracy Grimshaw unconscious after she was thrown from her horse at a north Sydney property on Thursday morning.

Witnesses said the 53-year-old presenter had been riding the horse, named Wombat, when it became spooked and went into a powerful high-speed gallop, The Daily Mail reported.

The incident resembled a rodeo as the bolting horse suddenly turned and threw her off "like a ragdoll", The Daily Mail reported witnesses as saying.

Grimshaw was unconscious for 15 minutes before being treated by paramedics, according to reports.