The Liberal and Nationals parties did not field a contender ahead of Saturday's vote in the Melbourne seat, leaving the Greens and Labor to battle it out.

"It's really whether you pick shooting or poison," federal minister Michael Keenan told the Nine Network on Friday, when asked whether he wants the Greens to win.

Labor frontbencher Brendan O'Connor, who campaigned with candidate Ged Kearney on Friday, said the Liberals had taken a cynical decision not to run in the seat.

"It's pretty rich from a Liberal minister to attack the candidates of this electorate when they won't run one themselves," Mr O'Connor told reporters.

Greens Leader Richard Di Natale said the minister was "insulting" voters.

"In a democracy, people are entitled to express their views at the ballot box and they shouldn't ever feel denigrated for making those choices," Senator Di Natale said.

Betting markets have the Greens' Alex Bhathal as favourite to take the seat off Labor, giving the minor party two lower house seats in federal parliament.

However, Labor frontbencher Anthony Albanese is confident his party will retain Batman.

"People have an opportunity ... to have Ged Kearney, a voice in government, or they have someone who can sit back and wait until decisions are made and then decide whether they will protest or not," he told the Nine Network.

The by-election was triggered by the resignation of sitting Labor MP David Feeney over his dual citizenship.

Mr Feeney, who held the seat with a one per cent margin, would have lost in 2016 had the Liberals not preferenced him ahead of the Greens.

Australian Associated Press