'Georgia' was working at the Australia Post business centre in Rosebank Avenue, Clayton South when she noticed the package, which looked similar to a VHS cassette, outside her building just before 5pm.

The woman correctly suspected the container, covered in stickers of $50 notes, belonged to a bank and took it to Springvale Police Station.

A Victoria Police spokeswoman said Georgia was "very shocked" when it was opened in front of her to reveal $100,000 in $50 bills.

Police are investigating how the item, used to refill an ATM machine, was left behind by a security company.

The money has been returned to the bank. Police praised Georgia, who has not yet been offered a reward, for handing in the stash of cash.

"It's good to see that there are people out there prepared to do the right thing," a Victoria Police spokeswoman said.