The province's budget watchdog says the Wynne government is being too optimistic with its projections on revenue and reducing the deficit.
The financial accountability office has done its own math and says that the government is forecasting "surprisingly" strong tax revenue growth, more than economists are projecting.
The Liberals have promised to balance the budget for the next three years.
The FAO agrees that can be achieved in 2017, but not the following two years, unless the province finds new tax revenue or decreases spending.
The watchdog says that the government's projections are based mostly on three taxes - the HST, personal income tax, and corporate taxes. But the FAO says they don't expect household expenditure, labour income, and corporate profits to grow as much as the Liberals say it will.
PC finance critic Vic Fedeli says the FAO's commentary shows the Liberals cannot be trusted.
"It's all about the election. They've made up numbers, their finances are a sham," Fedeli says.
Finance minister Charles Sousa released a statement thanking the FAO for confirming they can reach a balanced budget in 2017-18.
"Our government has a strong track record of beating our fiscal targets. We do so by taking a prudent approach to fiscal planning and by tapering down private sector forecasts for the province’s real GDP growth," Sousa writes. "Using this approach we’ve beaten our fiscal targets 8 years in a row."