Caitlyn Fischer, from Australian city Bairnsdale in Victoria, was riding the 12-year-old gelding Ralphie in a one-star cross-country competition when the accident occurred at fence two.

Immediate medical attention was provided by the on-course paramedic and doctor but the teenager was unable to be revived.

Equestrian Australia chairwoman Judy Fasher said the horse hit its foot on the fence as it was jumping and fell onto Fischer.

"On behalf of the whole sport I would like to extend our deepest sympathy to Caitlyn's parents Alisa Carr and Mark Fischer and her family," said Fasher.

"The thoughts and prayers of horse people all over the country are with Caitlyn's family and friends at this sad time."

Fischer, whose mother was at the event, was described as a young but experienced rider by Fasher.

"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.

Wayne Roycroft, who will conduct the investigation into Fischer's death on behalf of the International Equestrian Federation (FEI), said the barrier was only of a modest height and was usually used as a warm-up for the course.

Equestrian Australia has itself launched a full investigation into the accident, which comes only weeks after 17-year-old showjumper Olivia Inglis died while competing at the Scone Horse Trials in New South Wales.

Roycroft said the sport had its dangers, but the two incidents were just "freak occurrences".

"There's no real answer to how or why, they are accidents that happen," he said.

"Accidents do occur, to be honest it is part of our sport, if a horse falls, quite often a rider can be injured."

Fischer had recently moved to Melbourne to pursue her riding career, after spending 10 years as a member of the Bairnsdale Pony Club.

Her horse Ralphie is in a stable condition and remains under the observation of veterinarians.

"As a mother myself, my heart goes out to the family and friends of Caitlyn Fischer after this tragic accident," FEI secretary general Sabrina Ibáñez said.

"The safety and welfare of riders and horses is our top priority at all FEI events.

"On behalf of the FEI and the whole sport I would like to express our most sincere condolences to Caitlyn Fischer’s family and her many friends in the sport."

The rest of the trials have been cancelled.