The SA Electoral Commission will investigate claims a voter was handed a ballot paper with each box already numbered at a pre-polling centre for this month's election.

Sandra (not her real name) blamed a new computer system for queues "that went out the door" and a "level of chaos" which culminated in a pre-filled Lower House ballot allegedly being handed to her.

"I had to hand it back and say I want a blank one because this one has been filled in," she told ABC Radio Adelaide's Breakfast show.

"The response was: 'Oh, where did that come from?'"

Sandra said she had worked for the Electoral Commission, including as a scrutineer, and said she knew what it was like to be on the "other side of the desk".

She said the method of finding names in printed electoral rolls had been replaced with a computer system that took "five times longer".

"The tiny tablets people are working with are a nightmare and it takes so long for people to find you and mark you off, which is why we're getting these terrible queues.

"Those same people then have to hand back your filled-in ballot papers, which is a terrible mistake to set it up that way, then you have to put them into an envelope they print out and get you to sign it.

"I've never experienced anything so terrible and being handed that ballot paper was symbolic of the disaster that could occur. Things have to be fixed up."

'Mistakes shouldn't happen like this'

Deputy Electoral Commissioner David Gully said he was not aware of the situation and it would be investigated.

He said pre-polling centres in the past had relied on a person declaring their name verbally before they were looked up in a "street book".

Liberals Vickie Chapman and Steven Marshall voted at a pre-poll centre in Glynde. ( ABC News: Nick Harmsen )

"We're moving away from that into having something to look you up in and find your correct district, rather than just rely on what you say," Mr Gully said.

"People have to understand that declaration voting does take longer than voting on Saturday in the polls when you vote in your own district.

"The message I give to everyone out there — if you think you're going to beat the queues by going to a pre-polling centre, you're sadly mistaken."

There are 22 pre-polling centres across the state.

Senator Cory Bernardi, leader of the Australian Conservatives, said his past dealings with the Electoral Commission had been "unbelievably professional" but this was "really concerning".

"Mistakes happen, but they shouldn't happen like this," he told the Breakfast show.

"Giving back a ballot paper for them to seal and everything like that ... that's where you can get mischievous things happening or you can get mistakes like this.

"It will call into question a whole range of things if it's replicated anywhere."

Federal Labor MP Amanda Rishworth said Australia's electoral system had historically been "very accurate ... compared to stories we've heard in American presidential elections".

"I think the Electoral Commission should have a careful look at the claims Sandra has made."

Late on Wednesday, Deputy Electoral Commissioner David Gully told the ABC that the woman had never called him back — and he was unable to investigate or substantiate any of her claims.

He also said her comments raised a number of irregularities — including the fact that ballots are always folded after they're used, and that her background as both an electoral commission worker and a voting scrutineer did not add up.

"They're all required to sign a code of conduct, and that contains a declaration that they're not active politically, or they intend to be so, so a person acting as a scrutineer on behalf of a party or candidate would fail that declaration test, so there's just a couple of irregularities I would have liked to have an opportunity to speak to her about,” Mr Gully said.

He said despite the initial teething problems at Glynde and Mt Barker, pre-polling was going smoothly.