Luke Marsh had been handcuffed and confined to the back of the police van when he was then pepper sprayed for not giving up his phone during the ugly encounter with ACT Policing in May last year, which is now under investigation by professional standards.

An increasingly belligerent Marsh was later charged for kicking out at police, possessing an icepick, spitting at officers and destroying a mattress at the watch house.

But a court has blasted the police involved, saying the officer showed a "high level of impropriety" by arresting Marsh under the false pretence of being drunk and disorderly when really he wanted to question him about an earlier alleged assault.

There was little evidence he was drunk, and none to suggest he was disturbing the peace.

The "corner-cutting" police used what seemed to be the "easier option" to arrest him and then failed to caution Marsh before questioning him repeatedly, the court found.

Special Magistrate Jane Campbell said Senior Constable Julian Carey "created" the aggressive response in a "rightfully indignant" Marsh and set the scene for those later offences.

"Police should not resort to a power of arrest they find easier to administer, particularly in circumstances where the conditions for its exercise do not exist," Ms Campbell said.

"This is a serious misuse of the police powers to detain a person," she said of his conduct.

The magistrate ruled on March 9 that the arrest was unlawful and she dismissed all the charges against Marsh that had flowed from it.

Because the magistrate found the arrest was unlawful, she refused to admit the evidence from Senior Constable Carey or his partner that night Constable Lachlan Campbell.

Ms Campbell found Marsh guilty of damaging a photo frame holding a photo of himself earlier in the evening but not guilty of the common assault.

All other charges were dismissed.

Ms Campbell said the police had failed to meet the community standards expected from "those trusted with significant powers to interfere" with the right to liberty.

"There are strong public policy reasons that offences against police should be prosecuted.

"On the other hand, courts should not condone unlawful or improper conduct by police officers without compelling reasons to do so."

Chief executive of the Winnunga-Nimmityjah Aboriginal Health Service, Julie Tongs, said she was furious after watching the footage and that it was "disgusting" how Marsh had been treated.

What she saw was a member of her community being targeted by police.

"They knew who he was. He wasn't drunk. And we talk about trauma. How traumatising is that?"

"I see it all the time, and I hear about it all the time, but at least this time the magistrate's seen through the way they behave towards vulnerable Aboriginal people."

An ACT Policing spokesman said the matter was now with professional standards.

He said the organisation was working to address recent recommendations by the ACT Ombudsman about its systems and structures for Indigenous engagement.

"We recognise we can always do better when engaging with community groups and we are working to do so."

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