Tory Leader Brian Pallister says a government-commissioned study earlier this year touted new government infrastructure spending funded by the tax hike.
Pallister says the study did not take into account the effect of removing hundreds of millions of dollars from taxpayer pockets.
He says that money would have also created jobs and led to economic growth.
The Tories and the NDP government have been citing different statistics on unemployment, average wages and other figures as they argue over the impact of the tax increase.
A government-commissioned report from the Conference Board of Canada in March said the government's infrastructure spending will add $6.3 billion in economic growth over five years.
The sales tax was raised to eight per cent from seven last July, and the government has promised to use every cent to repair roads and bridges and build new flood-fighting projects.