State Greens leader Ellen Sandell said Labor's decision to run an open ticket in Prahran, in Melbourne's inner southeast, would ultimately benefit conservative candidates.
The preferencing move will mean Labor will direct voters to put its candidate Meghan Hopper first but will likely not advise how best to fill their remaining spots.
"Here's the modern-day Labor Party showing its true colours," Ms Sandell said in a statement shared with AAP.
"After months of scaremongering about a Liberal-One Nation government, Labor is now using its preferences to help elect one."
Prahran is held by Liberal MP Rachel Westaway, who narrowly wrested the seat away from the Greens at a 2025 by-election.
The minor party suffered a 13.4 per cent swing at the by-election in favour of the Liberals, after Labor opted not to run a candidate.
Labor preferences could be pivotal in allowing the minor party to win back the seat at the November 28 state poll.
"Labor voters will be frankly shocked that Victorian Labor would rather elect a right-wing Liberal-One Nation government, rather than elect a local, progressive Greens woman in Prahran," Ms Sandell said.
"This could be one of the most consequential elections in the state's history. The result may come down to a single seat."
Labor will also run an open ticket in the nearby blue ribbon seat of Caulfield, expected to be retained by deputy Liberal leader David Southwick.
It is understood Labor decided not to list preferences in either seat at the request of their local candidates.
"Our message is simple: put a 1 next to Labor candidates everywhere. We're fighting for majority government," a party spokesperson said.
Opposition Leader Jess Wilson on Tuesday announced plans to put Labor, the Greens and Victorian Socialists last on its how-to-vote cards.
She stressed, however, decisions would be determined on a seat-by-seat basis, subject to vetting of individual candidates.
Victorians will got to the polls in what the electoral commission expects to be the state's most complicated count.
If recent polling holds, the 12-year-old Labor government led by Premier Ben Carroll will likely be turfed in favour of the coalition.
A Resolve Political Monitor published by The Age on Tuesday put Labor's primary support at 22 per cent to the coalition's 30.
The survey of 1000 people in November pegged One Nation's share in second place at 23 per cent, ahead of the Greens at 13.
Leaders of both major parties are expected to hit pause on official campaign duties on Wednesday to join Remembrance Day commemorations.
