The funding, Harper said, would go to projects expected to create jobs and deliver quick results, most of it over the next three years.
The bulk of the spending — $2.8 billion — would go to improvements to historic sites, national parks and national marine conservation areas.
"We are ahead of track in terms of balancing the federal budget next year," said Harper, who made the announcement at a research facility in London, Ont.
"This gives us flexibility to make additional investments ahead of schedule in a wide range of necessary federal infrastructure projects."
The Conservative government has forecast a slim $1.9-billion surplus for next year. The projection was smaller than expected due to low oil prices and Ottawa's recent commitment of billions of dollars for a tableau of savings measures aimed at families.
The investments announced Monday included cash to build and repair schools on aboriginal reserves, to improve military facilities and to replace border infrastructure.
A spokesman for the Prime Minister's Office described the projects as "new funding" and said the money could start to flow as early as this year.
But Liberal deputy leader Ralph Goodale questioned Monday how the government could afford such an expensive set of new initiatives.
Goodale pointed to the government's recent announcement it would earmark about $2 billion per year for a controversial income-splitting proposal designed for families with kids, a plan the Liberals have promised to undo if they win next year's election.
"In doing that, they have gobbled up most of the available fiscal flexibility," Goodale said of Monday's announcement, which he called a "repackaging of things that already exist."
"If the government were truly making an announcement of $5 or $6 billion in new investments, that would almost require a budget lockup and the net impact of the announcement would be to put the country back into deficit again."
As an example, he said Harper's $500-million promise to fund on-reserve schools was in the government's spring budget.
In an email late Monday, a spokeswoman for the Finance Department said the majority of the money represents new funds that have not been previously announced.
"The exception are the funds that were recently announced to upgrade/refurbish the Canadian Science and Technology Museum, and the $500 million to build and renovate schools on reserve" announced in the 2014 budget, Stephanie Rubec said.
In its 2014 budget, the government said: "New funding to build and renovate schools is also confirmed, with $500 million over seven years beginning in 2015-16 for a new Education Infrastructure Fund."
The Liberal party has repeated its intention to make infrastructure spending one of its priorities, but it has offered few details about its plans.
Harper's announcement Monday included spending promises such as:
— $452 million to repair and improve Canadian Forces facilities;
— $440 million to replace border infrastructure;
— About $400 million to upgrade and build federal government buildings and other assets;
— $204 million in improvements for Via Rail infrastructure and federally owned and operated airports.
— $191 million to improve and repair heritage sites and museums.
"The full range of projects is simply too numerous for me to list here today," Harper said.
"These projects will deliver needed improvements to our federal assets and to our communities reasonably quickly. Second, from coast to coast to coast, in big cities and in smaller centres, these investments will provide jobs for Canadians now, and better opportunities for the future."
— With files from Andy Blatchford in Ottawa
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