Detectives were interviewing witnesses following reports of two robberies in the Collie CBD on Monday morning.
They were called to the first at the Collie Miners Credit Union just before 10.30am.
Minutes later they were called to Forrest Street following reports a man had robbed the BankWest branch.
"They had him on the ground and there was cash flying everywhere."
- Witness
Witnesses said they became aware something had happened when a woman stood outside the Forrest Street branch screaming "he robbed the bank".
One woman said a bystander then grabbed the alleged robber as he tried to get away.
"A man then just grabbed him and all the while I'm yelling to the other blokes outside the bank, 'help him, help him'" one woman said.
She said a second man then helped hold the man in question down until police responding to a duress alarm arrived seconds later.
Another woman told the Collie Mail: "They had him on the ground and there was cash flying everywhere."
The alleged robber hurled abuse at police officers and bystanders as he was loaded into the police van.
