An Adelaide mall in South Australia is set to commence extended weeknights trading on Tuesday next week as a city shop union workers called on retailers to only employ staff who wilfully volunteered to work additional hours.

The Rundle Mall Management Authority said on Thursday that new shopping hours until seven in the evening would mark the first revision in Adelaide's trading hours since 1994 as clamours from shop owners were reportedly pushing for late nights trading from Mondays through Thursdays as much of the city shops already serve clients until nine during Friday nights.

Up to 75 percent of Adelaide traders in Rundle Mall and around 50 percent from nearby shopping centres within the city were expected to commence extended trading hours after the public holiday next week.

However, a number of traders have expressed reservations that only a trickle of buyers would visit shops during the extended hours though Mall Authority chairman Theo Maras countered that tourists frequenting Adelaide were actually dismayed by its previous trading hours so the longer business shifts for the city shops should be a welcome change.

On the other hand, union secretary Peter Malinauskas expressed concerns that the new trading hours could affect shop workers' ability to spend quality time with their family as he urged retailers "to use volunteer staff rather than making people work unreasonable hours against their will."

Mr Malinauskas clarified that the union is not contesting the new trading hours, which they in fact were supporting, but the union would like to ensure that shop owners were fully aware that extended working hours could impact "on the lives of retail workers and we simply ask that employers take this into account when they adjust the new rosters."