Police officers who shot a man dead were confronted by a "highly stressful, dangerous" situation when he allegedly charged at them with a knife in a suburban home.
South Australia's Police Commissioner Grant Stevens has described the overnight shooting and backed the four officers involved for acting appropriately.
The state's police union says the force is under siege, understaffed and needs more resources to protect the community.
Police were called to reports of a mental health incident involving a man hurting himself with the weapon at Craigburn Farm in Adelaide's south just after midnight on Tuesday.
Mr Stevens said the 40-year-old confronted officers with a knife and when they unsuccessfully deployed their Tasers he continued to charge at them, so they opened fire.
Paramedics treated the man on the way to hospital where he later died.
"I can say that having watched the body-worn video that this was a highly stressful, dangerous situation," Mr Stevens told reporters on Tuesday.
"The police officers appear at this time to have acted in accordance with their training and taken the action necessary to protect themselves and the ambulance officers who were on the scene."
More than one of the police involved fired shots but the exact number of bullets involved is not yet known.
The police chief said he did not know why the Taser had failed but a blanket covering the man might have prevented its barbs acting as intended.
The man was alone at the time and his girlfriend raised the alarm after a video call where it became apparent he had harmed himself and was threatening to continue doing so, Mr Stevens said.
Police Association president Wade Burns said he had spoken with the officers involved, none of whom were injured during the incident.
"All the information I have so far indicates they acted in line with their training and took appropriate defensive action to protect themselves, their colleagues and the ambulance officers who were at the scene," he said.
He said officers were often confronted with critical life-or-death situations that required split-second decision-making.
"Police are the last line of defence and often dealing with people who are experiencing crisis situations," he said.
Detectives from the major crimes squad, specialists from the forensic response section and the force's internal investigation team are investigating what happened.
The incident follows a tough period for SA police during the festive season.
A female officer responding to a domestic incident in the days before Christmas was allegedly attacked and injured by a sword-wielding man, who has since been charged with attempted murder.
Two officers were allegedly assaulted, bitten and struck on Friday by a man who threatened supermarket staff and customers while brandishing a lighter and a can he claimed was filled with petrol.
Another six officers were reportedly assaulted and injured on Sunday night during a series of violent incidents and brawls in Adelaide city.
"Police officers in SA are under siege at the moment," Mr Burns said.
"They are doing their best to protect the community despite limited resources and relentless understaffing."
