A man has been arrested and charged over a skimming device that was discovered on a popular cash machine in Adelaide.

The card skimmer and covert camera were allegedly discovered on an ANZ automatic teller machine (ATM) on a street off Rundle Mall on Saturday morning.

A 31-year-old Victorian man has been charged with two counts of dishonest dealings with documents and dishonestly manipulating a machine.

Police said it appeared both the skimmer and covert camera were purpose-built to fit onto the machine, making it almost impossible to tell it was not part of the ATM.

Detectives immediately began investigating the discovery and will allege the devices had been attached to the ATM for a number of hours.

While investigating the device, police say they saw a man acting suspiciously in Rundle Mall near the machine, and after speaking with him, arrested him a short time later.

He was refused bail and is expected to appear in the Adelaide Magistrates Court later this week.