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.
Australian Conservatives leader and South Australian Senator Cory Bernardi told ABC Radio Adelaide 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 said.
"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."