There have been no fond goodbyes after the former leader of the South Australian Liberals changed sides this week.

Martin Hamilton-Smith led the Liberal Party in SA for several years, but has now declared himself a Liberal independent, taking the Trade, Defence Industries and Veterans' Affairs portfolios in the fourth-term Labor administration of Premier Jay Weatherill.

A vandal scrawled “Deserter. The community won't forget. Your time will come”, in red paint over Hamilton-Smith’s electorate office on Tuesday night, showing the level of discontent from some sides.

But the now independent Liberal has been welcomed to the Labor government fold, by Premier Jay Weatherill at least.

“He'll assist the Labor Government in building a bridge to the business community,” Mr Weatherill said.

Political analyst Professor Clem Macintyre said it was a good thing for Labor to have “a very able former opposition member sitting in their Cabinet”.

The response from Liberal party members has ranged from mildly incensed to pure vitriol.

SA Liberal leader Steven Marshall cancelled travel plans at the airport to hold a press conference to condemn his former colleague.

“What we’ve seen today is the true mark of the man... his values don't sit at all well in that Labor Cabinet, he signed up to our Liberal policies, our Liberal philosophies.”

Federal Education Minister Christopher Pyne railed against the perceived treachery.

“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,” he said.

Liberal Senator Cory Bernardi said voters in Hamilton-Smith’s electorate of Waite would be disappointed at the decision to “jump ship and join the other team”.

Senator Simon Birmingham agreed, saying “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,” he said.

But Mr Hamilton-Smith is resolute, and says plenty of his voters have backed the move.

“If the Liberal Party want to have a discussion with me about loyalty, teamwork and treachery I may have a lot to say about it,” he told the ABC.

“In regards to some of the comments being made by Mr Marshall and Liberals, since I’ve now heard those comments let me simply send them this message - there are some very serious problems within the Liberal Party.”