Liberal Senator Simon Birmingham says he is certain former South Australian Liberal leader Martin Hamilton-Smith has no chance of retaining his Adelaide seat at the next state election.

Mr Hamilton-Smith has switched his support to the Labor Government and taken a ministry, which Senator Birmingham says is the highest act of betrayal towards voters in the Waite electorate.

The former Liberal says he is now a Liberal independent and Labor has given him the portfolios of Trade, Defence Industries and Veterans' Affairs.

Senator Birmingham says the voters of Waite will not accept the defection.

"Waite is an electorate that consistently votes Liberal, it's an electorate that voted very, very strongly for the Liberal Party at the last election," he said.

"His voters will see him as a rat and I have no doubt that the voters in Waite will want the first opportunity to vote against him.

"All this is about is Martin getting to be a minister again and it's very, very sad. There's nothing about what Jay Weatherill and the Labor Government stand for that is consistent with Martin's beliefs or views or anything he's claimed that he's wanted to achieve in politics."

Another Liberal Senator, Cory Bernardi, has rejected Mr Hamilton-Smith's reasons for switching support.

"We've supported him over successive elections, he was a former leader of the Liberal Party, of course we're disappointed and the people of Waite should be disappointed too because you can't just jump ship and join the other team and claim it's the national or public interest because it's not," he said.

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SA Liberal leader Steven Marshall was at the airport yesterday and about to fly overseas when his now ex-colleague phoned him to drop the political bombshell.

He cancelled his travel plan and called a news conference, condemning the MP as a traitor.

"Martin has some great skills but what we've seen today is the true mark of the man," he said.

"His values don't sit at all well in that Labor Cabinet, he signed up to our Liberal policies, our Liberal philosophies."

Mr Marshall says people in the blue-ribbon southern Adelaide electorate should make their feelings clear to their local member.

"They need to tell Martin that they didn't vote for him to join a Labor Cabinet, there is nobody that went to that poll [last March] thinking 'I'm voting for Martin, he might join in and back the Weatherill Government who he's been out there really attacking, for the last 12 years'," he said.

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Federal Education Minister Christopher Pyne agreed the MP was a traitor in Liberal eyes.

"Martin Hamilton-Smith will go down, not as a fighting leader of the Liberal Party who believed in things but as the greatest traitor to his political party in the history of the state," he said.

"Everyone respects someone who fights the good fight and loses, no-one respects a political traitor.

"Labor MPs will be laughing at Martin Hamilton-Smith behind their hands when he is in the room and laughing out loud at him behind his back."

Graffiti sprayed on electorate office

Someone has taken out their anger overnight at Mr Hamilton-Smith's suburban electorate office, spraying it with graffiti.

The message across the front doors and windows in red paint reads: "Deserter. The community won't forget. Your time will come".

Political analyst Professor Clem Macintyre says Mr Martin Hamilton-Smith's switch of support consolidates South Australian Labor's grip on power.

"Barring resignations or deaths and by-elections... we wouldn't expect the numbers to change and Jay Weatherill would be much more confident that he is securely in office for the period through to 2018," he said.

"With Martin Hamilton-Smith I think they've got a very able former opposition member sitting in their Cabinet."

Premier Weatherill has spoken of his hopes from Mr Hamilton-Smith's backing for Labor.

"He'll assist the Labor Government in building a bridge to the business community," he said.

The other independent given a Labor ministry, Geoff Brock, does not think the regional support package Labor pledged him will be affected by it now having a bigger voting buffer in the House of Assembly.

Sorry, this video has expired SA Liberal defects, will be part of Labor cabinet

"I've been able to achieve $39 million for job opportunity projects, my focus is on creating jobs," he told reporters.

"I'm up here in Port Augusta and the upper Spencer Gulf to talk to councils etcetera as part of my regional development portfolio, but certainly I'm looking forward to working with Martin Hamilton-Smith in the new Government."

A former senior Labor staffer, Jill Bottrall, says Mr Weatherill has strengthened his Government with the political manoeuvre.

"This is just one of the most astonishing things I have ever seen, to actually go out there and secure a member of an opposition and bring them into your Cabinet that's stunning," she said.

SA has fixed election dates provided a government runs its four-year term.