The Electoral Commissioner has delivered a blow to the Liberal Party just days before the South Australian election, upholding a complaint of misleading advertising over Opposition Leader Steven Marshall's claim his party's energy plan would save households $300 per year.

Key points: SEC ruled savings would be made whether Liberals' plan implemented or not

SEC ruled savings would be made whether Liberals' plan implemented or not The advertisements were found in breach of the Electoral Act

The advertisements were found in breach of the Electoral Act A complaint against Labor was also upheld for "misleading" advertising

The claim was made by the Liberals in advertisements and robocalls to thousands of South Australian households when the party launched its policy last October.

But modelling released by the party showed that most of the $300 saving would be delivered whether or not the policy was in place, forcing Mr Marshall to backtrack the day after the policy's release.

The Commissioner responded to a complaint from the Labor Party yesterday, saying the Liberals' advertisements were inaccurate and in breach of the Electoral Act.

"From the information available to me it appears the plan includes measures that are already in place and that a significant part of the savings referred to would result from those measures," the Commissioner Mick Sherry wrote.

"It also appears that a significant part of the savings referred to (around $230) will eventuate whether the plan is implemented or not."

The Labor Party was also called out for misleading advertising over claims that "the Libs have a secret plan to cut $557 million of the GST share from South Australia".

The Commissioner upheld a complaint made about the flyers by the Liberal Party, and asked Labor senator Alex Gallacher to "immediately cease distribution" of the flyer and publish a retraction online.

SA Labor swiftly published Senator Gallacher's retraction on social media, admitting the statement was "inaccurate and misleading".

Marshall maintains Liberal Policy the 'best plan' for energy

The Liberal energy policy includes an investment in a new interconnector to New South Wales, but the modelling undertaken on the party's behalf showed the bulk of reductions in power prices would be delivered before an interconnector would be built.

Official modelling shows power prices are tipped to fall in coming years, driven by the installation of new renewable energy plants in South Australia and Victoria.

Mr Marshall said the Liberals "don't accept" the party had mislead voters and maintained that the Liberal policy would reduce energy prices.

"We're in correspondence with the Electoral Commission regarding the letter that they've sent to us today, but we believe that we have got the best plan for energy at this election and we'll be promoting it right through until 6:00pm on Saturday afternoon," he said.

"The reality is when the Liberal plan is fully implemented there will be a reduction in prices here in SA.

"The Electoral Commission is not disputing the $302 figure — in fact, we're the only party that has put our policies up for independent verification and published the ... report."

He said analysis showed the Liberals' policy would reduce the wholesale energy price by 24 per cent more than what was otherwise forecast to happen.

Mr Sherry indicated he had written to Mr Marshall and Liberal Party State Director Sascha Meldrum requesting they publish a retraction and correction by way of a media release, on social media and in conjunction with the Liberal plan wherever it is published.

Labor complained to the Electoral Commissioner about the Liberal advertisements last year, and the Liberal Party complained about the Labor advertisements in February.

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Xenophon will 'fight' ruling on Fiat Bambino signs

SA Best leader Nick Xenophon is also in hot water with the Electoral Commissioner over the slogans appearing on his campaign vehicle.

The Commissioner took issue with the size of the material and said it exceeds the 1 square metre allowed.

"I write to advise you to remove the car from public view immediately," Mr Sherry wrote in a letter.

But Mr Xenophon took the vintage Italian car to Newton Shopping Centre in protest.

The size of the slogans on Nick Xenophon campaign car attracted criticism from the Electoral Commissioner. ( ABC News: Tom Fedorowytsch )

"How is it that the major parties, the Australian Hotels Association can run massive campaigns, spend millions of dollars, but a Fiat Bambino is somehow in breach of the law?" he said.

"We don't say it is, we think that we are within the law, but if we are prosecuted we will fight this all the way."

He said the slogans were not too big, but the Constitution also implied his right to political expression and he would defend the car in the High Court.

Labor was also in trouble earlier in the election campaign over the size of slogans on the so-called "Jay Bus".