An elderly bank customer had his account emptied by an opportunistic thief after he left his bank card in an automatic teller machine in Melbourne's north.

The 81-year-old used his bank card at an ATM in Edwardes Street, Reservoir about 3.30pm on October 11, and forgot to take his card from the machine.

Police say that as the elderly man walked away, the thief – who was next in line – emptied the man's bank account.

Realising his error, the elderly man returned within seconds to search for his card, but the thief dashed off after stealing the money, slipping the card into his pocket.