The Liberals are fighting "every election to win" but Malcolm Turnbull and John Howard say it will be tough for the coalition to win any of the July 28 by-elections.
The government is running in three of the five by-elections on Super Saturday, but the prime minister and former leader played down their chances.
"The last time the government won a by-election from the opposition was 100 years ago," Mr Turnbull told reporters in Sydney on Saturday.
"We fight every election to win, but you have to be realistic."
Liberals believe they have their strongest chance in the Tasmanian seat of Braddon, where former MP Brett Whiteley is running against Labor's Justine Keay.
Mr Howard said the government's chances in Braddon and the Queensland seat of Longman are "very remote".
"I think it's very, very hard and tough and unlikely. People don't normally award by-elections to incumbent governments," Mr Howard told reporters.
"I think people who are saying otherwise are really trying to tamper with expectations. The expectations must be that Labor will retain both Longman and Braddon."
Georgina Downer is running in the South Australian seat of Mayo, and Mr Howard said she also faced a difficult battle against independent Rebekha Sharkie.
Parliamentary Speaker Tony Smith issued the writs on Friday for five by-elections, the first time so many federal by-elections have been held on the one day.
Electoral rolls in Perth, Fremantle, Longman, Braddon and Mayo will close on June 22.
Nominations will close on July 5 with the declaration of nominations a day later.
Four of the by-elections were triggered by MPs facing dual citizenship questions, while Labor's Tim Hammond retired from the seat of Perth for family reasons.
