They followed the fleeing offender down Queen St and got in touch with police who were able to trace him via CCTV.
When officers caught up with him, Papa was in a car park trying to change out of his bloody clothes in a bid to evade arrest.
Meanwhile, the victim was being rushed to hospital with life-threatening injuries.
Though police argued Papa should be sentenced to a term of imprisonment, Judge Russell Collins decided that unlikely have a positive effect.
Despite two previous assaults on his criminal record - one against police - Papa was given 320 hours community work and 18 months intensive supervision.
The twist came when defence lawyer John Kovacevic spoke about his client's efforts to organise a restorative justice conference.
It could not go ahead because the victim was found to be an overstayer and after recovering in hospital, he was imprisoned, Mr Kovacevic said.
After the hearing police said the victim was understood to still be in the country but could not be located to complete a victim impact statement.
Judge Collins said the sentencing had caused him "a huge amount of concern" and he was far from confident he was applying the right sentence.
"As a father of three sons, one of the greatest fears I had was them being out at night and being king hit in a cowardly and thuggish way by someone like you," he said.
Nevertheless, the judge said he could not ignore the fact Papa had stable employment as a driller and worked hard.
Judge Collins was keen to stress the seriousness of the charge of injuring by an unlawful act.
"You caused bodily harm to someone and if he had died, you would have been guilty of manslaughter," he said. "This was a cowardly, brutal act."
Police said the victim made a full recovery.
