The federal government ran a $2.3-billion deficit in August but Ottawa remains in surplus over the first five months of the fiscal year.
Finance Canada released its monthly tracking of Ottawa's revenues and expenses Friday and the Fiscal Monitor report showed a monthly deficit for the first time this fiscal year.
Conservative Leader Stephen Harper had regularly pointed to last month's Fiscal Monitor report during the election campaign as evidence that federal finances were healthy and that the Liberal plan to run deficits was unnecessary.
The report Mr. Harper pointed to showed a $5.2-billion surplus over the first four months, but the deficit in August has shrunk the size of this year's surplus to $2.8-billion. That is still above the $1.4-billion surplus the Conservatives projected in the April budget.
The monthly surplus figures to date are somewhat at odds with what economists are expecting in light of Canada's economic performance to date.
The Conservative government's April budget assumed the Canadian economy would grow by 2 per cent in 2015 and 2.2 per cent in 2016.
The latest consensus forecast of private sector economists is for the economy to grow by 1.2 per cent in 2015 and 2 per cent in 2016, according to a survey by FocusEconomics.
As a general rule of thumb, Finance Canada has said that a 1-per-cent reduction in Gross Domestic Product growth would likely reduce federal revenues by at least 4 per cent. Inflation is also a key factor for government revenues.
The Parliamentary Budget Officer is expected to release an updated economic and fiscal forecast shortly after the new Liberal government is sworn in on Nov. 4. The PBO report will not attempt to estimate the economic or fiscal impact of Liberal campaign promises.
The new government will likely release a fall economic update that will outline how economic changes since April will influence Ottawa's bottom line.
The Conservative budget forecasted small annual surpluses over the coming years. However the Liberal Party won the Oct. 19 federal election on a platform that promised to fund new infrastructure spending by running three years of deficits worth a combined $25-billion before returning to surplus in 2019-20.
The Liberals argued infrastructure spending will boost economic growth and that the increased debt is manageable due to current low interest rates.
Friday's Finance Canada report shows low interest rates have both a positive and negative impact on federal finances. Part of the August deficit was due to the fact that Ottawa had to contribute more money to finance public service pensions in light of low interest rates. However, the federal government also saved money on debt servicing costs.