In a heated exchange at a people's forum hosted by Sky News and The Advertiser in Adelaide on Wednesday night, Mr Marshall said the Liberal Party will release its campaign costings document on Thursday, which will include a "modest" efficiency dividend.

He said the Liberals wanted to "protect all frontline services" but would not rule out cutting the jobs of teachers and nurses.

"There's a cut actually envisaged in the forward estimates and yes, the Liberal Party will be preserving that," he said.

He said there would be no cut to the overall health budget, and any efficiencies would be reinvested into the system.

A Labor pledge of support for young tradies and a Liberal promise of more trips to Kangaroo Island were what South Australia's major parties hoped would define them on one of their last campaigning days to convince voters to back them at the ballot box.

Those expectations were dashed when the state's Electoral Commission found both parties had made misleading claims in their campaign advertising.

Electoral commissioner Mick Sherry said a claim made by Mr Marshall that the Liberal's energy policy would save South Australian households more than $300 was misleading, as many of the savings would occur regardless.

Federal Labor Senator Alex Gallacher was also "inaccurate and misleading" when he claimed in a flyer that "the Libs have a secret plan to cut $557 million of the GST share from South Australia", the commissioner found.

In separate letters to the Liberal and Labor camps, who levelled complaints that led to the findings, Mr Sherry said he had asked each party to publish retractions.

The call was quickly heeded by Senator Gallacher but not Mr Marshall.

He opted not to withdraw his comments, first made in October, but said his party was "looking at the issue" and was in dialogue with the commission.

The issue gave Premier Jay Weatherill - who said Senator Gallacher's materials had "nothing to do" with his team - an opportunity to rebuke his rival.

"He has been misleading, he has been incompetent and South Australians can not afford to risk their future on Steven Marshall," he told reporters.

Mr Marshall again refused an opportunity to back down from the claim at Wednesday night's people's forum.

SA-Best leader Nick Xenophon did not escape a slap on the wrist from the electoral commission, which said he had breached the Electoral Act for having too much advertising on a Fiat Bambino campaign car.

Mr Xenophon said he thought parts of the Act may be in breach of the constitution and the hard-to-miss little, red car would not be stood down before Saturday's election.

"The Fiat Bambino will be around right until the close of polling," he said.

Australian Associated Press