Lorenz Martin Daley, 20, fronted the ACT Supreme Court on Wednesday after pleading guilty to two counts of common assault over the attack, which one of the victims described as "animalistic, vicious and horrific".

Police documents tendered to the court state that Daley was "tripping" on LSD when he accosted an unsuspecting couple as the pair sat in a car in Gowrie in late December.

He then repeatedly punched the victims while boasting "look at me, I'm a stallion" and shaped up in a boxing stance with both fists balled.

In March this year, Special Magistrate Jane Campbell dismissed the charges and recorded no convictions.

"I am satisfied that Mr Daley has well and truly learnt his lesson and is unlikely to reoffend," she said.

In a July hearing, the ACT DPP's deputy director, Anthony Williamson, said the appeal was based on the following grounds: that the magistrate did not properly assess the objective seriousness of the offending, she found the drug use as a mitigating factor, she found "considerable consequences" would flow from convictions when the evidence did not support this, and other appeal decisions showing harsher penalties for similar offending.

The appellant argued that convictions would help other institutions know about his offending and that the initial sentence was "so plainly short of the appropriate sentences as to bespeak error of principle".

In response, Michael Kukulies-Smith for Daley said at the time his client's drug use "appears to be an aberration" and that there was "naivety as to the impacts".

Mr Kukulies-Smith said as a legislative matter, a recording of a non-conviction was unnecessary to making an offence reportable.

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In his decision on Wednesday to uphold only the ground of appeal for convictions, Justice John Burns said the "failure of the magistrate to impose convictions makes the sentences imposed manifestly inadequate".

He said evidence before the magistrate "established that the respondent was within the class of typical young male offenders who commit offences of violence while intoxicated".

"The offences committed by the respondent were serious offences of their type," Justice Burns said.

"The failure to record convictions in such cases erodes the deterrent effect that sentences for offences of violence should be expected to incorporate.

"There was no cogent basis for the magistrate to proceed without recording convictions."

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