TORONTO -- Ontario's opposition parties are hoping former premier Dalton McGuinty can shed some light on the massive destruction of emails when he returns Tuesday at committee hearings into cancelled gas plants.

McGuinty testified last month that he made the decisions to cancel gas plants in Oakville and Mississauga at a cost of at least $585 million, far above the $230 million he and the Liberals had originally claimed.

However, after his testimony, the privacy commissioner reported that top Liberals in McGuinty's office deleted emails on the gas plants, despite legal obligations to retain their records.

Progressive Conservative Leader Tim Hudak says he hopes McGuinty will use this second opportunity "to tell us what really happened."

Hudak says people would believe McGuinty much more if he were "honest with taxpayers" and explained why documents were destroyed.

The New Democrats say the privacy commissioner's report "cast a tremendous amount of doubt about the honesty of the (former) premier."

Privacy Commissioner Ann Cavoukian will also appear at the gas plant hearings Tuesday, just before McGuinty's second appearance, to talk about her investigation into the mass deletion of government email accounts.

The Ontario Provincial Police launched a criminal investigation earlier this month after complaints by the Conservatives about the destruction of emails by senior Liberal staff.

"I think premier McGuinty knew and I think Premier Wynne knew about the destruction of email documents. I have no doubt about that," said Hudak.

"It's not something that happens by accident. That happens as part of an organized campaign to hide the footprints."

McGuinty has denied he had a hand in the deletion of emails.