Liberal Leader Justin Trudeau says his government would reverse the Conservative plan to charge tolls to drivers using Montreal's new Champlain Bridge.

Construction of the new bridge, replacing the rapidly aging structure that currently connects the city to the South Shore, is already underway.

"The Champlain Bridge is a vital piece of infrastructure, not just for Quebecers who use it every day, but for Canada," Trudeau said during a campaign stop in Brossard this morning.

"Decisions about its future should not be taken lightly."

The bridge is expected to cost $4.23 billion.

The Conservative government has said that, in order to help offset the costs, it would require drivers to pay a toll.

According to a 2014 report compiled by the parliamentary budget officer, the new Champlain Bridge needs a toll of between $2.60 and $3.90 for each passage if it's to pay for itself,

The decision has been widely criticized by the Conservatives' political opponents, who argue a toll system would increase congestion. The NDP has argued a toll would hit lower-income families and workers who commute to work in the city the hardest.

Montreal's mayor Denis Coderre, a former Liberal cabinet minister, has also voiced his opposition to the toll plan.

"It's not a new bridge. It's a replacement bridge, and it's infrastructure that is part of the Canadian economy," Coderre said, flanked by Trudeau at a news conference at Montreal City Hall.

Coderre announced his own election wish-list yesterday that included investments in social housing and public transit, as well as $77 million a year for infrastructure, with funding based on municipalities' population densities.

Last week, Trudeau announced a plan to spend $125 billion on infrastructure over the next decade, nearly double the Conservative commitment.

Trudeau said the plan would create long-term economic growth but would lead to three years of deficit spending before balancing the budget in 2019.

Bridge scheduled to open in 2018

Engineering giant SNC-Lavalin has the contract to build the bridge, slated to open in 2018.

The new Champlain Bridge will span the Saint Lawrence River in one long stretch. (Infrastructure Canada)

The Champlain is the busiest bridge in Canada. Montreal's bridge corporation estimates nearly 60-million vehicles drive over it each year.

But the structure is in bad shape. An inspection report released in July found cracks, splits in support cables, corrosion and surface deterioration on the steel-truss cantilever bridge.

The bridge corporation insists the 53-year-old Champlain Bridge is still safe to drive on. At least $127 million have been allocated for repairs in 2015 alone.