Caitlyn Fischer, from Bairnsdale, fell during a cross-country competition at the Sydney International Horse Trials on Saxony Road, Horsley Park, around 10:40am.
Equestrian Australia chairwoman Judy Fasher said the horse hit its foot on the fence it was jumping and fell onto Ms Fischer.
“I don’t know how it actually happened, but the horse fell and fell on the rider,” Ms Fasher said.
The teenager was treated at the scene but was unable to be revived.
Ms Fasher said the teenager’s mother was at the event and her father and brother are flying to Sydney now.
“It was shocking for everybody involved,” she said.
“The thoughts and prayers of horse people all over the country are with Caitlyn’s family and friends at this sad time.”
She said Ms Fischer was a young, but experienced rider.
“We scrutinise very carefully the level of experience that riders have and she would not have been allowed to enter this competition unless she had met the credentials that the sport sets out,” she said.
Ms Fasher said the fence where the fall happened was not a complex obstacle.
“Our course designers always try to make the first few fences on a course like this, very simple, so they are easily understood by both horse and rider,” she said.
“She would have started off at a good pace, but not a fast pace.”
The rest of the trials have been cancelled.
‘Quietly spoken, delightful girl’
Ms Fischer had recently moved to Melbourne to pursue her riding career, after spending 10 years as a member of the Bairnsdale Pony Club.
The club’s district commissioner Janine Bromwich said the Bairnsdale community had been rocked by the news.
“Caitlin would have to be one of the most gorgeous girls you’d ever wish to meet,” Ms Bromwich said.
“Quietly spoken but just bright, just a delightful girl.”
Ms Fasher said Equestrian Australia had launched its own investigation into the incident and would also be cooperating with a police investigation.
Ms Fischer’s horse Ralph is in a stable condition and remains under the observation of veterinarians.
Fairfield Police are investigating and will prepare a report for the coroner.
Second death in recent months
In a statement, organisers of the event said Ms Fischer was riding in a one-star competition when the “tragic accident” happened at fence two on the course.
Wayne Roycroft, from the International Equestrian Federation, said the barrier was only of a modest height and was usually used as a warm up for the course.
Another young equestrian, 17-year-old Olivia Inglis, was killed in March in another NSW horse trials accident.