A racehorse has been put down and a jockey is in hospital after a hail storm hit during the running of a thoroughbred race in country NSW.

The storm swept across the Grafton track during the running of the fourth race at the NSW northern rivers track on Thursday.

All runners finished the event despite hail falling as the field entered the home straight.

But when the jockeys eased their mounts down and tried to return to the mounting enclosure, drama unfolded.

Three jockeys and the clerk of the course were thrown from their horses.

Jasen Watkins injured his shoulder and is in Coffs Harbour Hospital while the clerk of the course Alan Watkins suffered facial cuts.

The racehorse Beau Brock, which finished second, took fright and suffered head injuries when he ran through a fence and had to be put down.

"We went onto the track and obviously we could see the storm coming," steward Drew Smith told Sky Thoroughbred Central.

"It was just raining ever so lightly but within the space of a minute it went to torrential hail so you couldn't have seen it coming.

"I've never seen anything like it, it was relentless."

The remaining six races were abandoned.