The Defence Department has admitted breaching its duty of care to a young soldier fatally shot in the head during a live-fire exercise, saying it was a terrible and avoidable tragedy.
Victorian soldier Private Jason Challis, 25, died after he was shot in 2017 at the ADF's Mount Bundey training area, about 120km southeast of Darwin.
The department pleaded guilty in the Northern Territory Local Court on Tuesday to failing to comply with its health and safety duty to Pte Challis.
Defence's lawyer Fiona McLeod SC said it was recognition of "the fact that Private Challis was not protected while in service".
"(This) was a terrible and avoidable tragedy and one that should never have occurred and is deeply regretted," she said.
Pte Challis was shot in the head and knee during an urban operation exercise in a mock town.
He and three soldiers approached a wood and hessian structure to engage dummy targets inside it.
Two servicemen entered the building and fired from within it at a target that Pte Challis was crouching behind outside the structure.
"His head wound was immediately non-survivable," prosecutor Jennifer Single SC said.
Major General Matthew Pearse said Pte Challis had a right to expect a safe working environment.
"Our system failed Jason Challis," he said.
"On behalf of Defence, I apologise unreservedly for our failure. This is not something we ever want to repeat.
"We've made a number of changes to the way we conduct our business."
The ADF investigated nine serving members closely linked to the incident, one was sacked and six others were reprimanded.
The court heard "vagaries of the human factor" were found to be a key cause of the incident, along with the "deliberate disregard" for and a "momentary lapse" from the procedures.
"Army identified that there were several individuals directly related to the safety aspects and command of the exercise that failed," Maj Gen Pearse said.
The much-loved 5th Battalion rifleman and his team were supposed to have moved as one to prevent the soldiers from firing on one another.
Three safety supervisors were observing when the incident occurred but failed to notice Pte Challis had not come from the rear to the front of the building, according to a coroner's report.
Despite investigations by the NT coroner and the NSW police forensics unit, and two inquires by the ADF, it is not known why he became separated.
Pte Challis's mother, Helen Brandich, told the court she had tried to stay strong but the matter had "taken way too long to be finalised".
"I cry behind closed doors," she said in her victim impact statement read to the court.
"The last four years have been the hardest years of my and my family's life.
"Before my son was killed he was the life of the party. Now he is just a memory.
"His death has sucked the life out of all us and left a huge hole in our hearts."
Pte Challis's father, John Challis, told the court his son was a "true Aussie digger", labelling his death a "tragic f*** up"."
"Jason died serving his country," he said.
"He may not have been at war or in Afghanistan but he had his whole heart in this and he died doing something he absolutely loved.
"Not many people can say that."
Defence was facing three counts of failing to comply with health and safety duty after an investigation by the work, health and safety regulator Comcare.
But two charges were dropped and one amended.
The maximum penalty is $1.5 million.
The case returns to court on Wednesday.