Michel Samson said in a report released Tuesday the province could have trouble keeping to the $1.75-billion deficit the government is projecting in 2014-15.
That means the deficit could soar if government spending is not reined in when Finance Minister Carlos Leitao tables his 2014-15 budget on Wednesday.
Samson says budgetary revenues are expected to fall $485 million short of the projection contained in the budget tabled in February by former Parti Quebecois finance minister Nicolas Marceau.
He also believes it will be a major challenge for the government to keep a lid on program spending on the next few years.
The Liberal government ordered a review of the PQ budget, a document that was not adopted because the sovereigntist party called a general election this spring.
Premier Philippe Couillard did not seem surprised by the contents of Samson's report.
It "shows the scale of the challenge we're facing," Couillard told a news conference. "But we will be up to the challenge."
Despite the grim news, Couillard promised that Quebec will post a balanced budget in 2015-16.
The premier also said Wednesday's budget will include measures to boost employment. He did not elaborate.
Couillard has warned since taking power in April that the time has come for hard economic decisions.
The government has already brought in a hiring freeze in the public sector and is doing a complete review of its programs and services.
Treasury Board president Martin Coiteux will also table the spending estimates for 2014-2015 on budget day.
Leitao, meanwhile, showed off a new pair of running shoes to reporters in Quebec City, a switch from the dress shoes usually worn before a finance minister tables a budget.
Leitao said the $279 runners, which he paid for himself, reflect the efforts the new Liberal government will make to try to relaunch the Quebec economy.
These include a new maritime strategy as well as a return to the previous Liberal government's plan to develop northern Quebec.
The Liberals were elected with a strong majority on April 7, winning 70 of the 125 seats in the legislature.
The minority PQ government lasted only 18 months.