Voters in the Melbourne seat will head back to the polls on Saturday, less than a year after the last federal election, following the resignation of Liberal MP Alan Tudge.
But Australian Electoral Commissioner Tom Rogers said pre-poll figures were well below where they were at last year's poll.
He said there was a worry not as many people would turn out at Saturday's poll, with voting numbers down by five per cent.
"By-elections have traditionally had less people participate than in a full federal election but it's just as important to have your say," Mr Roger said.
"Typically, if someone casts their vote early in one election they'll do so in the next one as well - and we're not quite seeing that here which makes us worry about low participation."
The commission has sent out more communication to voters for the upcoming by-election than they have for any previous by-election.
So far, more than 14,000 people had cast their votes at early voting centres as of Monday evening, down from the more than 18,540 voters at the same point last year.
There have been more than 17,500 applications for postal votes submitted with the commission.
The Liberals hold Aston by 2.8 per cent, after the party suffered a more than seven per cent swing against it at the last election.
Despite the small margin, both parties are downplaying their chances at the poll.
A sitting government has not won a seat off the opposition at a by-election for more than 100 years.
Former union organiser Mary Doyle will run in the seat for Labor, having previously campaigned for Aston at the 2022 federal election, while the Liberals have picked Melbourne City councillor Roshena Campbell as their candidate.
Mr Rogers said voting in by-elections was compulsory.
"Failure to vote may result in a fine and will mean you have no say on who represents you in federal parliament."
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