A disability pensioner was pinned down by several police officers, beaten with a baton and doused in capsicum spray in his Melbourne front yard.
The incident caught on CCTV and aired on ABC television on Tuesday shows about half-a-dozen officers took part before the man was sprayed with water from a high-pressure garden hose.
"We need all employees within our organisation to be accountable for their actions and face the consequences when poor behaviour or misconduct is identified," Victoria Police said in a statement responding to the footage.
Police had gone to visit the man, known as John, last September to do a mental health welfare check.
During the incident, an officer can be heard saying: "F***ing idiot, do you like that? Do you like that? Smells good, doesn't it?"
John responds, "Oh my back, my back."
An officer films him being sprayed with the hose and John can be heard in the footage saying: "You happy?... how tough are youse (sic)?"
The incident left him "humiliated and degraded" according to Robinson Gill Lawyers, which is representing him.
He had not committed a crime and has not been charged, they said in a statement.
Fearing reprisal, he sought legal representation before making his complaint against police, the firm said.
The case is one of many showing the police complaints system is broken and requires an independent body to investigate police misconduct, it said.
"Many of our clients are left frustrated and disenfranchised by the police complaints system," the firm said in a statement.
"In many instances, clients who have submitted substantial evidence regarding allegations of egregious misconduct have had police determine their complaints as being unfounded or not substantiated.
"In our experience, the findings of police investigations do not correlate with the outcome of civil proceedings."
A statement by Victoria Police says it has no tolerance for poor behaviour within its ranks and will examine any new evidence carefully.
"We want people to have absolute confidence that when they make a complaint against police, it will be investigated thoroughly and with integrity," the force said.
