She was unable to be revived, despite first aid being administered and paramedics being brought to the scene in a Westpac helicopter. The inquest heard she was not conscious after the fall and did not suffer. Her horse was put down after sustaining a fractured neck in the fall.

About seven weeks later, on April 30, 19-year-old Caitlyn Fischer died seconds into her cross-country run at the Sydney International Horse Trials when her horse's front feet hit the second jump.

The horse then fell on her, causing head injuries and killing her instantly.

Caitlyn's mother, a registered nurse who has worked in intensive care, sprinted onto the course and was the first to reach her daughter's side. She was immediately able to see that Caitlyn had passed away.

On Monday, a two-week inquest into the deaths opened at Lidcombe Coroner's Court in Sydney. It will examine nine issues, including whether safety procedures at equestrian events are adequate, and whether physical aspects of the courses contributed to the deaths of Caitlyn and Olivia.

Counsel assisting the inquest, Dr Peggy Dwyer, said there is no doubt the three-pronged sport of eventing – comprising dressage, show jumping and cross-country – is a "challenging and dangerous sport", but one of the aims of the inquest will be to minimise the risk of injury.

Dr Dwyer said there has already been "significant change introduced to the sport in NSW" including the use of collapsible pins in certain jumps, so they effectively fold in and collapse if they are struck by a horse. This reduces the chance for a "rotational fall", the type of fall Caitlyn and Olivia were involved in.

In April 2017, a year after the deaths, Equestrian Australia, the governing body for the sport, also employed a safety officer for the first time.

Dr Dwyer said both Caitlyn and Olivia were "beautiful young women" who were "clever and vibrant". They were highly experienced and competed in eventing at an elite level.

"There is a common thread in relation to both Caitlyn and Olivia that they were both young women with enormous potential, both intellectually, personally, and of course also in this sport," Dr Dwyer said. "Both of them were also delightful young women."

The inquest is expected to hear that Olivia's mother Charlotte Inglis walked the cross-country course with her daughter on the morning of her fall, and had concerns about "a number of jumps" including a combination jump, 8A and 8B, which had a downward approach.

She was so worried she spoke to another rider, Olympic champion Shane Rose. Olivia went ahead with her ride, and died after her horse fell on jump 8B.

Caitlyn's run on the cross-country course was delayed for 10 minutes after another rider fell. Her accident, 210 metres from the start of the course, was captured on video, which will not be made public out of respect for her family.

Deputy State Coroner Derek Lee extended his personal condolences to the girls' families and friends, who were present in court.

"I remain deeply saddened by your tragic losses," Mr Lee said.

In a statement read out to the inquest, Equestrian Australia CEO Lucy Warhurst said the organisation is "deeply sorry for the loss" of the two young women and seeks to provide the safest possible environment for its riders.

"Both Olivia and Caitlyn are sorely missed by the entire equestrian community," Ms Warhurst said.