Stack, who was delisted by the Tigers in 2022, was out celebrating his cousin's 21st birthday when he was involved in a scuffle, swinging punches against another man.
The former Tiger was handed a suspended $500 fine after pleading guilty.
Stack's verdict comes just days after he was charged with a string of drink driving offences, set to face court next month for charges including drink driving, driving without a license and driving an unregistered vehicle, as well as two counts of driving against a red traffic light.
Speaking outside court on Friday, the 22-year-old claimed he was acting in self-defence during the altercation in Northbridge.
“Oh I've just got people that are trying to bring me down mate — I've just got a lot of people trying to bring me down. Got nothing else to say,” he told 7 News.
Troubled former @Richmond_FC player Sydney Stack has fallen foul of the law again. The 22-year-old has been hit with a $500 dollar fine after swinging punches at a man in Perth’s nightclub district. He claims it was self-defence. https://t.co/5zYfOfGqUb #7NEWS pic.twitter.com/FqTW9bV4YB
— 7NEWS Melbourne (@7NewsMelbourne) January 13, 2023
The 22-year-old's career was marred by off field controversy, sent home from Richmond's Covid hub in Queensland in 2020 after breaching the league's Covid-19 safety protocols and entering into an altercation with a member of the public.
Stack was also charged for for breaching COVID lockdown rules again late in 2020 after travelling to Western Australia and failing to comply with quarantine restrictions.
After being delisted by the Tigers last year, Stack has been linked to a possible return to the WAFL to play for the Perth Demons.