SA Libs won't take Hamilton-Smith back

South Australia's opposition leader has ruled out letting defector Martin Hamilton-Smith join a Liberal government if the party wins the next state election.

Mr Hamilton-Smith defected from the Liberals to become an independent cabinet minister with the Labor government in 2014.

But he says if he holds the balance of power after SA goes to the polls in 2018, he'll negotiate with both parties.

"Well Martin, thanks for the offer but I won't be taking you up on that," Liberals leader Steven Marshall posted on Facebook on Thursday.

"If Martin is re-elected in Waite and holds the balance of power, then Labor will be continuing on in government."

"So if you live in Waite and want a Liberal Government you'll need to vote for our Liberal candidate!"

But Mr Hamilton-Smith labelled Mr Marshall's comments "extraordinary" and said they proved he was "just on another planet".

"What he's saying is he'd rather stay in opposition and refuse to let me back in the party rather than form government," Mr Hamilton-Smith told AAP.

"If my vote was the deciding vote, I think common sense would tell you what the answer would be."