All Adelaide shops will be able to open on Sunday mornings under legislation passed by South Australian parliament on Thursday.

Changes to the Shop Trading Hours Act voted in by the lower house will allow stores to open from 9am on Sundays, two hours earlier than the current rules allow.

Shops will still have to close at 5pm on Saturdays and Sundays.

Most stores, except big supermarkets, will be allowed to trade on Boxing Day, bringing them into line with CBD shop trading.

Boxing Day trading was first allowed in Adelaide's suburbs in 2018 under a special exemption granted by former treasurer Rob Lucas.

Suburban shops will not be allowed to open on any other public holiday but CBD stores will continue to be allowed to trade from 11am to 5pm on certain public holidays — except for Good Friday and Christmas Day — and from noon until 5pm on ANZAC Day.

The legislative changes also allow shops to trade until midnight on Black Friday, and on three weekdays before Christmas.

The Liberal Party promised to deregulate shopping hours altogether before being elected in 2018 but was unable to get it past the upper house of parliament.

Labor was then elected earlier this year promising to allow Sunday morning and Boxing Day trading.

Industrial Relations Minister Kyam Maher said he anticipated the changes will come into effect in the next few weeks.

"It puts into legislation some things that have been a feature for the shopping calendar," he said.

Mr Maher said the laws struck a balance between the needs of businesses, shoppers and workers.

"It will see the greatest reform to shop trading hours this century," he said.

Opposition spokesman Matt Cowdrey said the changes should have gone further.

"What we saw through this process to date is a baby step forward to what needed to be much bigger reform to bring South Australia into the 21st century," he said.

Opposition leader David Speirs said he gave a "big tick" to earlier trading on Sunday morning, but said shopping should be allowed on some other public holidays as well as for a "couple of hours" extra on Saturday and Sunday evenings.

"We think this is a huge, missed opportunity here in South Australia to respond to what South Australians want in terms of slightly more flexible opportunities to go out to the shops," he said.

The Shop, Distributive and Allied Employees' Association, which represents retail workers, welcomed the changes but wanted to ensure retail workers were not required to work on these days against their will.

Under the rules, staff have to agree to work on Sundays or public holidays.