He told Ms Bradley to look after the unconscious Japanese tourist while he followed the offender and called police.
Mr Comyn kept his distance from the attacker, and relayed information to the police about where he was going.
"He wasn't running. He was just cool and calm, like he'd done absolutely nothing wrong.
"I kept my distance ... a couple of times he turned around and I just pretended I was on the phone talking to someone else.
"I just kept him in my sight as far away as I could because I had no idea if he'd come after me."
The mugger rifled through his victim's bag before casting it aside, and Mr Comyn picked it up.
For 10 minutes he followed the man as he walked down Wyndham St and Queen St to Shortland St then along High St. At the end of High St, the man met a small group of friends. Not long after that, police caught up and made an arrest.
Inspector Kerry Watson said without Mr Comyn police would not have caught the offender.
"A Good Samaritan followed the offender through town in heavy rain, providing updates for the police," Mr Watson said.
"There's been quite a few robberies in town, but they just blend in with other people."
Ms Bradley looked after the mugging victim, saying his face was bloodied and swollen.
"His English wasn't so good, he was by himself and was a visitor."
The tourist was taken to Auckland City Hospital with minor injuries, and a man is to appear in the Auckland District Court on aggravated robbery charges.
