As extended Rundle Mall trading on weeknights starts next week, an Adelaide shop union says retailers should make up their rosters from those staff who volunteer.

The Rundle Mall Management Authority said 7:00pm closing would be the first significant change to trading hours since 1994, when shops were allowed to open on Sundays.

It said there had been widespread support among retailers to open later from Monday to Thursday.

Many city shops have long opened until 9:00pm on Fridays.

It is expected that about three-quarters of Rundle Mall traders and more than half of those in the city's big shopping complexes will start longer trading from next Tuesday, the day after the public holiday.

Some traders have expressed concern that there will be too few shoppers in the early evenings on early nights of the week.

But Mall Authority chairman Theo Maras said research showed tourists, among others, were disappointed with current trading hours in the city.

"Our city is saved from being the doughnut of nothing after 5 or 5.30, which unfortunately we've been branded by other interstate and overseas cities," he said.

The secretary of the shop assistants' union Peter Malinauskas thinks retailers should only roster staff who volunteer for extra hours.

"A number of retail workers are concerned about working later in the evenings, which jeopardises their capacity to spend time with family, particularly young children, in the evenings," he said.

"We simply ask that retailers use volunteer staff rather than making people work unreasonable hours against their will.

"The union isn't opposed to retailers using these additional hours, we're surprised that they haven't done it earlier, but the point remains the same - trading these additional hours is fine under the law, but it does have a consequence on the lives of retail workers and we simply ask that employers take this into account when they adjust the new rosters."

You really are very generous crook, with other people`s money.