"A search after his arrest revealed the man indeed had an imitation Glock firearm hidden in under a tea towel."
Manurewa Police Area Commander Inspector Julia Lynch said the callers did "a fantastic job".
"They did all the right things," she said.
"They called police, kept an eye on what was going on and when we arrived, stayed around to give us statements - we couldn't be happier with how this played out.
"This is prevention - when public and police work together like this.
"I am also proud of how swift my staff reacted to resolve the situation."
Anyone who witnessed what happened outside the shop is encouraged to call Crimestoppers anonymously on 0800 555 111.
A 21 year-old man will appear at Manukau District Court on Friday charged with unlawfully carrying an imitation firearm and preparing to commit a crime.