The 19-year-old was taken to hospital in a critical condition with two gunshot wounds to the abdomen following the incident at Arana Hills, in Brisbane's northwest, on Wednesday.
The shooting occurred after three officers attended the welfare check for the teenager about 12.30pm.
"The call for assistance came from his mother who was present during this incident, regrettably," Detective Superintendent Andrew Massingham told reporters.
"Mum is distressed. She is traumatised."
The man had previously received mental health treatment, he said.
The officers were speaking to the man at the Akebia St property when he allegedly pulled out the knife.
"As a result there has been two shots fired by one of the male officers," Det Supt Massingham said.
A taser was also deployed "at some point" during the incident, which will be the subject of an Ethical Standards Command investigation with oversight by the Crime and Corruption Commission.
"The timing of that in terms of when the shots were fired will be subject of the ongoing investigation," Det Supt Massingham said.
Officers provided first aid before he was taken to the Royal Brisbane Hospital, where he was undergoing surgery late on Wednesday.
A first year constable fired the shots, police confirmed.
"It is traumatic for all involved and that young man is feeling it right now," Dept Supt Massingham said of the constable.
"As police, we come to work each day, not expecting to have to use this level of force against people and, in short, it is our last option."
Police say they have established a crime scene at the home while investigations continue.
Lifeline 13 11 14
beyondblue 1300 22 4636
18°C