NDP Leader Andrea Horwath is calling for a public inquiry into the gas plants controversy. (Canadian Press )

The New Democrats said Friday the discovery of more documents on cancelled gas plants in Oakville and Mississauga shows Premier Kathleen Wynne has not been truthful, and demanded a public inquiry into the controversy.

The Liberals stunned Ontario's opposition parties Thursday by releasing hundreds of more pages on the cancelled energy projects after insisting for months that all the relevant data had been made public in two separate batches last fall.

The government was warned last November by the Ontario Power Authority that there "likely" would be more documents unearthed, but Liberals — including Wynne — insisted as recently as Wednesday that they'd all been released last fall.

"She has clearly not been truthful already, because the deputy minister was aware of this back in November, and I don't believe for a minute that everybody else wasn't aware of it as well," said NDP Leader Andrea Horwath.

"There's no way that the premier and everyone on down didn't know about this, and there's no way that the new premier wasn't briefed about this."

Speaking in Ottawa Friday, Wynne again rejected a public inquiry and said a legislative committee and an investigation by the auditor general would get to the bottom of the cancellations and the release of the documents.

"It's entirely possible to get all of the information that we need to get out through the mechanisms that are already in place," said Wynne.

"I really don't think that spending another $20 or $25 million do put in place a third or fourth option is necessary."

The Progressive Conservatives said the third batch of gas plant documents released Thursday were proof of an "immense amount of political interference" by the Liberals, and agreed that Wynne must have known there were likely more documents coming.

"She would have to know as far back as last year as a senior cabinet minister," said Conservative Jim Wilson.

"It's hard to believe there'd been nothing said in the premier's office on a decision of this magnitude, such a huge political decision."

Horwath said she would not go back on her word to support the Liberals' throne speech because of the latest developments in the gas plants saga.

But Horwath warned the legislature could become seized with the gas plants issue again as it was last fall when Dalton McGuinty suddenly prorogued the house and announced his resignation as premier.

"Unless there's something to coverup, you don't bother with a coverup," she said.

"So it's obvious that this is one steaming mess that needs to be looked at with a separate, unbiased set of eyes through a public inquiry."

The OPA said Thursday it "messed up some search terms" in its first two looks for files using Liberal government code names for the gas plants such as Project Vapour, Project Banana and Project Fruit Salad.

The Tories and NDP insisted for months that the Liberals had not released all the gas plant documents, and said Friday they were convinced there were still hundreds of missing pages.

"We still do not have any correspondence between OPA officials and the premier's office, and have only limited documents about Project Vapour, Apple, Fruit Salad and Banana," said Wilson.

"And we still have yet to see any emails or correspondence from the premier or the minister of energy."

The opposition parties believe the cancellation of the gas plants cost taxpayers a lot more than the $230 million the Liberals claim, a figure the officials from the Ontario Power Authority refused to confirm.