Residents within the Ceduna District Council region still have an opportunity to cast their vote in the supplementary council election.
As of Tuesday, a total of 710 votes (33.59 per cent) had been returned.
Voting material must be received by 12pm on Monday, while the council office also has a ballot box to vote.
The counting will take place from 1.30pm on Tuesday.
South Australian electoral commissioner Mick Sherry encouraged Ceduna voters to make sure they completed their ballot and returned it in the envelope provided to the council office as soon as possible.
"To be counted, your completed envelope must be received no later than noon this Monday, 9 December. If you haven't had a chance to put it in the mail yet, your best option now is to drop it in the ballot box at the council administration office on O'Loughlin Terrace - just make sure you do it before noon on Monday," he said.
"Earlier this year I conducted an election for the Mid Murray Council and the successful candidate won by a margin of just two votes.
"This was another example of how every single ballot really does matter, so don't delay - get your envelope down to the council office today. Your vote may well decide the outcome of the election."