An engineering expert believes the Oakville gas plant cancellation cost taxpayers $638 million, instead of the $40 million the Liberal government is standing by.

Bruce Sharp testified at committee hearings Wednesday at Queen's Park.

NDP MPP Rosario Marchese says there's no way the Liberals don't know the actual cost of breaking the gas plant construction  contracts.

But Premier Kathleen Wynne says she's been honest, maintaining the numbers she's mentioned in public are the only figures she knows of.

Wynne underlines this is why she wants the Auditor General to look at both cancellations, as well as the committee conducting the hearings.