A shadow cabinet of eight has been unveiled by South Australian Opposition Leader Steven Marshall.

There previously had been 15 members in the shadow ministry, which now has been cut to 12.

The new line-up is Mr Marshall, his deputy Vickie Chapman, Iain Evans, Martin Hamilton-Smith, Rob Lucas, Stephen Wade, David Pisoni and David Ridgway.

Mr Hamilton-Smith has been given the economic and regional development portfolio and Ms Chapman will handle transport and infrastructure.

Mr Evans remains treasury spokesman and has a new cost of living shadow portfolio.

Mr Lucas is the new health spokesman.

Four other Liberal MPs make up an outer ministry, Dan van Holst Pellekaan, Michelle Lensink, Duncan McFetridge and Steven Griffiths.

Mr Marshall said the reshuffle was merit-based.

He said a smaller shadow cabinet would allow more efficient decision-making.

"That's the whole focus of this, to get a more effective team working on better outcomes for people in South Australia," he said.

Mr Marshall said some portfolios would be delegated to members of his shadow cabinet because the outer ministry members would not attend those shadow cabinet meetings.

"This is not something new, it's not some sort of Marshall plan, this is used extensively right throughout parliaments throughout the world," he told reporters.

"It's the same example, same situation that is used in Canberra."

Former party leader Isobel Redmond went to the backbench when Mr Marshall took the helm this week, as Parliament resumed.

The new leader said Ms Redmond had declined the offer of a shadow portfolio.