Centre Alliance Mayo by-election candidate Rebekha Sharkie says about 400 of her posters have disappeared over the past two days.

Key points: Hundreds of Mayo candidate Rebekha Sharkie's posters go missing

Hundreds of Mayo candidate Rebekha Sharkie's posters go missing She says in many cases the posters have been replaced by corflutes for her opponent Georgina Downer

She says in many cases the posters have been replaced by corflutes for her opponent Georgina Downer The Liberal Party has denied it is involved

Ms Sharkie stopped short of blaming supporters of her opponent, Georgina Downer, but she said Liberal corflutes often appeared soon after hers vanished.

"I have no idea where they've gone but I know that in many of the cases they've been immediately replaced with a different poster — a blue one — which makes it even more frustrating, particularly if we've got photos or volunteers telling us that they were there at midday and then at 1:30pm they're gone and another one is in its place," Ms Sharkie said.

Voters in the Adelaide Hills and Fleurieu Peninsula seat will go to the polls on July 28.

The by-election was called after Ms Sharkie resigned in May following a High Court ruling that ACT senator Katy Gallagher was ineligible to contest the 2016 federal election.

Both women had failed to renounce their British citizenship before the election.

Ms Sharkie beat Liberal Jamie Briggs in the 2016 election.

Ms Downer, the daughter of former foreign minister Alexander Downer, was preselected as the Liberal candidate for the by-election.

Ms Sharkie said her volunteers would be checking today to see how many posters — each worth $5 — had vanished.

"That's one form of advertising, if you are a minor party, that most people receive because I just don't have thousands of dollars to stuff people's letterboxes with information about me," she said.

"[It's] just to let people know an election is on — you'd be surprised how many people I spoke to don't know a by-election is on."

Unlike in some other states, in South Australia election posters can be put up on power poles, however there are some restrictions, such as not being too close to intersections.

They are not allowed on most sign poles.

A Rebekha Sharkie poster on a pole near a roundabout in Victor Harbor. ( Facebook )

Liberal Party denies any involvement

In a statement, Liberal Party state director Sascha Meldrum denied Ms Downer's campaign had any involvement in the posters' disappearance.

"This is another example of Rebekha and Labor trying to smear the Liberal Party with false claims," Ms Meldrum said.

"We're focused on the things that matter for the people of Mayo, whereas Rebekha and Labor are focused on lies."

In response to a Facebook by Ms Sharkie complaining about her posters going missing, other people have commented with pictures of Ms Sharkie's posters placed illegally.

Similar issues came up last election, with state Liberal MP Stephan Knoll posting a picture of a Nick Xenophon poster on a tree.

A Nick Xenophon poster on a tree in Mayo in 2016. ( Twitter: Stephan Knoll )

At the March state election, Adelaide Liberal MP Rachel Sanderson was accused of putting up her posters too early.

Ms Sharkie said she believed it was other people who had moved her signs rather than her volunteers putting them up in the wrong spots.

"It's just some people playing silly games," she said.

"I don't think my volunteers have time for that and we certainly haven't been contacted by authorities to say they were in the wrong place."

Victor Harbor Council denied it had removed any posters despite at least one picture showing a Rebekha Sharkie sign placed on a light pole next to a roundabout.

Other Mayo candidates have much fewer election posters.