Sheena Lee Harvey, 20, of Shervill Court, Portland, pleaded guilty to six counts of theft.

Portland Magistrates Court heard she was a long-standing employee of the city's Safeway who was given the role of service supervisor after years of strong performance.

About 6pm on May 7 this year a loss prevention officer saw Harvey take an unknown quantity of $50 notes from one of the supermarket's registers and return to the staff room, where she hid the money in her purse.

The 20-year-old was already under surveillance by Safeway due to significant end-of-day shortfalls on registers she used, with more than $1000 unaccounted for.

She was approached by the officer and made full admissions, handing back the money - found to be $200 - before being interviewed by police.

On May 28 she was captured on closed-circuit television taking $100 from a customer's wallet that was kept in a locked service counter cupboard, prompting her immediate termination.

Defence counsel Amanda Chambers said her client, who stole a total of $1845, had no prior criminal history and was deeply ashamed of her actions.

She said Harvey had been under pressure to support her mother, who was between seasons as a vineyard worker but reluctant to apply for government benefits.

The Portland resident had secured another job since leaving Safeway but no longer had any control of money, Ms Chambers said.

Magistrate Stephen Myall convicted Harvey, fining her $1500 and ordering that she pay $1745 in compensation.