Leader Angus Taylor was alongside candidate Raissa Butkowski as she cast her vote in Lavington, north of Albury, in NSW on Saturday.
"I'm going to be fighting till six o'clock tonight to make sure we do (win) and that's my focus today," Mr Taylor told reporters.
"Working with Raissa, we're going to be talking to the people of Farrer and making sure they understand our commitment to the important issues in this area."
The race for Farrer in southern NSW is being viewed as an electoral test of support for Pauline Hanson's populist One Nation party, which could win its first lower-house seat.
The electorate has been held by either the Liberal or National party since its inception in 1949, and was held by recent opposition leader Sussan Ley from 2001 until she was ousted by Mr Taylor in a February leadership spill.
Ms Butkowski thanked Ms Ley for her 25 years of serving the people of Farrer and said the former leader had contacted her following her preselection.
"It's really important the people of Farrer have somebody who's going to fight, not only for now, but far into the future," Ms Butkowski said.
One Nation candidate and polling frontrunner David Farley was mobbed by a press pack entering Albury Public School shortly after voting began at 8am.
If he wins, a historic first lower house federal seat for Pauline Hanson's party could spell trouble for the coalition as it continues to lose votes to the alternative conservative party.
"The nation's looking for change and Farrer's screaming out for change, so today is very, very important," Mr Farley told reporters.
"I've got three clear jobs to do: I've got to win today, I've got to make sure we're ready for the next election and I want to help Pauline Hanson and her team build a powerful political party for Australia."
He and independent Michelle Milthorpe appear neck and neck to take the seat but analysts believe preferences could get the populist party over the line, while coalition candidates struggle.
Attending a voting centre in Jindera, Ms Milthorpe rejected criticism from opponents over funding received from Climate 200.
"It's just lazy rhetoric by parties who are scared and threatened by people who want to do things differently," she told reporters.
"At the end of the day, everyone needs to be funded for their campaigns.
"I'm open and transparent about where mine comes from and my positions are informed by the people of Farrer."
Ms Milthorpe said politicians didn't need to be negative or aggressive to get things done.
"You just need to be prepared to listen to and learn from people," she said.
Nationals leader Matt Canavan on Friday insisted other regional coalition seats were not at risk of falling to One Nation, arguing there were unique circumstances at play in Farrer.
"By-elections are always different ... it's a totally different race than normal," he said.
While Mr Farley and Ms Milthorpe have appeared close in the polls, preferences from coalition voters will give the One Nation candidate an edge, RedBridge pollster Tony Barry told AAP.
"With the coalition preferencing One Nation, they are the intuitive choice to win the seat."
If One Nation wins, it will show the party is a strong electoral force, particularly in regional areas, the former Liberal strategist said.
"If they cannibalise the Liberal and National Party vote as expected, then it's a very safe conclusion to say that we're going to see similar voting behaviours and patterns in other regional seats."
Labor isn't running a candidate in the by-election.
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