Sarah Jane Minter learnt that the hard way when she pocketed $300 cash she found in an ATM - in full view of CCTV cameras.

The high-quality footage proved to be the Warrawong mother of four's undoing, as it depicted her taking the $300 from the dispenser and stashing it in a baby bag hanging from a pram.

She also made a transaction to check her bank balance, which police subsequently tracked.

Yesterday, Wollongong Local Court magistrate Michael Stoddart was scathing of the opportunistic crime, saying it was a "weak act" to steal the money just after an absent-minded shopper accidentally left it in the ATM.

"We are talking about $300 ... this is just a weak act," he said.

"This woman might have really needed the money, you know nothing about her."

The court heard Minter, 31, had a number of offences on her record, including theft and assault occasioning actual bodily harm, among others.

She was convicted of one count of larceny, placed on a 12-month good behaviour bond, fined $500 and ordered to pay $300 compensation.