Bank staff had activated a silent alarm after the robber fled - forcing the bank into lockdown.
"I was only in there a minute. The tellers said someone had robbed the bank and the doors got locked."
He was locked inside the bank for about 10 minutes - which "seemed forever" - before police had arrived.
Josh said he did not notice anything peculiar about the robber until a teller told him what had happened.
Police said the robber had handed over a note, demanded money from a teller before snatching the note up and taking off with the money.
Police in marked and unmarked police cars, including dog handlers, converged on Greerton Village after the alarm was raised and spoke to bank staff and witnesses.
Another man said he raced to the bank where his wife worked when she called him after the robbery.
"She rang home just to say before anyone heard the news that it happened and that everyone was okay ... She was calm enough and knowing everyone is safe is something."
Greerton Village Mainstreet manager Victoria Thomas was shocked, saying it was the first robbery at the shopping centre in her three-and a-half years in the job.
"I feel so sorry for the bank staff caught up in this. I hope they catch the buggers. It's not on."