Ontario's information watchdog says it "strains credulity" to believe staff in the premier's office didn't know they were supposed to keep e-mails on cancelled gas plants in Oakville and Mississauga.

The NDP asked Information and Privacy Commissioner Ann Cavoukian to investigate after senior staffers in former premier Dalton McGuinty's office testified they deleted all their e-mails on the gas plants.

Cavoukian says she's baffled that such senior officials wouldn't know they had a legal requirement to keep all such documents for several years.

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The commissioner says she was not pleased – "putting it mildly" – and promises her office will release its report into the deleted email accounts next month.

A frustrated Cavoukian told reporters lawyers are "muzzling" her from commenting on the slow and staggered release of thousands of other gas plant documents until her report is released.

She said her office "left no stone unturned" in its investigation, but apologized for being unable to say more about the gas plant documents.

The opposition parties say the Liberals spent at least $585-million to cancel the gas plants to save Liberal seats in the 2011 election, and accuse the government of trying to hide the true costs of the decisions.