Premier Kathleen Wynne says it will be up to Ontario's auditor general to determine exactly how much it will cost taxpayers to cancel gas plants in Oakville and Mississauga.

The opposition parties were fuming after Energy Minister Bob Chiarelli said that the estimated 40-million-dollar cost of cancelling the Oakville project "could be wrong."

Wynne and Liberal cabinet ministers have been using the figure for months, but the new premier admitted the cost could be higher when the auditor delivers his report.

She says the report "may contain differences" with what she and other Liberals have said in the past about the costs of scrapping the gas plants.

The Conservatives and New Democrats have long maintained the cost of cancelling the gas plants is much higher than the Liberals admit, and could top one billion dollars.

In addition to the auditor general's investigation, the legislature's justice committee is also probing all aspects of the government's decisions to cancel the gas plants to save Liberal seats in Oakville and Mississauga.