Evan Greenwood was meant to meet his parents and brothers for lunch.
The Sydney family were on a ski holiday at Charlotte Pass, in the Snowy Mountains, on September 6, 2010, when Evan – who was autistic – failed to meet them as planned.
Rescuers searched kilometres of stark white snow, and his body was found the next day near the Snowy River.
After Evan's death, relatives described him as a "beautiful person who loved life and cherished every minute".
Five years later the family, Evan's father Wayne Greenwood, his mother Jeanette Keir, and his older brothers Keith and Brian, say they continue to suffer serious psychological trauma.
They are suing Charlotte Pass for negligence, alleging the ski resort exposed Evan to a "risk of injury and death of which they knew, or ought to have known", according to a statement of claim filed in the NSW Supreme Court.
"During the morning of September 6, 2010, the deceased was skiing by himself in the resort and whilst in weather conditions of very poor visibility, he mistakenly skied outside the boundary area of the resort," the court document said.
"Due to adverse conditions the deceased mistakenly skied into the Snowy River and shortly thereafter died of exposure and hypothermia.
"The death of the deceased was occasioned by the negligence of [Charlotte Pass]."
The statement of claim alleged Charlotte Pass failed to have a proper system to mark the edges of the designated ski field, and that boundary ropes were too high, allowing skiers to inadvertently pass underneath.
The family said they suffered "nervous shock" and an array of psychological conditions, which have built up medical expenses and restricted their ability to work and earn money.
Their "amenity and enjoyment of life has been restricted," the document said.
In its defence filed in court, Charlotte Pass denied it had full control over the activity of everyone within the ski area, and denied that the weather was poor that day.
Charlotte Pass argued it had bright orange poles, ropes and discs marking the boundary areas, along with signs prominently displayed around the resort warning of the risk of injury or death.
"The deceased did not suffer injury or death by resort skiing. The ... death was occasioned by the deceased and by the [Greenwoods] failing to ensure that the deceased conducted himself in a safe and controlled manner," the defence document said.
It also said the family failed to supervise Evan, and allowed him to ski alone when his condition meant he suffered "heightened anxiety" in unfamiliar situations.
Charlotte Pass said its signage warned that skiing and snowboarding carried a "significant risk" of injury.
Signs told skiers they were responsible for staying in control on the slopes, observing warning signs, and keeping off closed trails and runs.
The matter returns to court in January.