OTTAWA -- Opposition parties say that a plan to shift more than $2 billion worth of federal infrastructure spending points to problems with one of the government's cornerstone programs, despite what the Liberals say.

The cash comes from planned spending in multiple years and multiple funds set up by the Liberals and the previous Conservative government, as well as large-scale projects, such as the Champlain Bridge replacement in Montreal.

More details are expected some time next week when departmental reports are tabled in Parliament.

Conservative infrastructure critic Michael Chong said he is hearing concerns from cities that they don't have enough time to spend their transit and water money, pointing to a need to change the terms and conditions on the Liberal infrastructure cash.

"This is not just a cash flow issue, this is actually impacting real infrastructure decisions that are being taken on the ground," Chong said.

He also said the delay conveniently helps the government "with its ballooning deficit, something that they're clearly worried about."

NDP critic Matthew Dube said the figures should give the government some pause as it negotiates longer-term agreements with provinces and territories to make sure the money delivers the social and economic benefits the Liberals have promised.

"We can brag about how much money is being invested into infrastructure, but at the end of the day, if the shovels aren't in the ground, and the projects aren't happening ... you can't start looking at that upside."

Infrastructure Canada's website shows that as of last Friday, there was about $20.5 billion left unspent across 13 different programs, including two set up by the Liberals. More than half the money, about $12.8 billion, is from the marquee infrastructure fund the Tories set up.

The two Liberal funds -- one for public transit projects, the other for water systems -- have together doled out about $82 million, leaving about $5.2 billion unspent.

The federal government regularly has to carry over infrastructure money from one fiscal year to the next: Spending analyses have shown about one-quarter of infrastructure funds don't get spent in the year for which they are budgeted.

Federal dollars only flow once project proponents submit receipts for reimbursement, often leaving a lag of weeks, months or years between when work takes place and when infrastructure money is actually spent. Projects can also be delayed for reasons beyond the control of the federal government, such as bad weather or a labour disruption.

The Liberals have already carried over $828 million to this fiscal year because it couldn't be spent in the 12 months leading up to April 1.

Infrastructure Minister Amarjeet Sohi said Tuesday that the majority of the approximately 4,000 projects the government has approved for funding since November 2015 are underway, based on the information cities and provinces have provided the Liberals -- even though federal payments so far are a fraction of what's budgeted.

Sohi said the government's plan to move funding to later years has more to do with cash flow management than it does with the state of infrastructure work.

"We're the last ones to pay out because that's how the system works," Sohi said. "If you visit various communities where these projects are being funded, you can actually see the work being done."

The Liberals promised in the last election to move unspent infrastructure money into the gas tax fund that goes directly to cities for transit, water or roads projects.

The government closed out several old infrastructure programs at the end of March, giving the gas tax fund $30.1 million of money that provinces didn't earmark for any projects.

That won't happen in this case because the money comes from programs that still have years left before they expire.