TORONTO -- Ontario's opposition parties say they do not believe Don Guy, Dalton McGuinty's former chief of staff and long-time Liberal campaign director, who testified Tuesday he had no role in the decisions to cancel gas plants in Oakville and Mississauga.

In a combative appearance before the justice committee's hearings into the cancelled gas plants, Guy started arguing with NDP house leader Gilles Bisson even before his opening statement.

"Mr. Bisson, you made a false allegation about me," Guy said.

"Like usual, you're trying to be the bully, sir, and I'm not going to accept that," Bisson fired back. "Once a bully, always a bully. He tries to bully the committee."

The exchange was typical of Guy's two hours under oath, which often saw him talking over top of New Democrats and Progressive Conservatives on the committee, frequently repeating their questions back rather than answering them.

Guy testified that McGuinty made the decision as Liberal leader to scrap the gas plants prior to the 2011 election, at a cost of at least $585 million, and that his job was only to arrange the public announcement of the cancellations.

"Were you involved in the discussion to cancel the gas plant," asked PC energy critic Vic Fedeli.

"I don't want to parse words, but what do you mean by the discussion," responded Guy, one of the most powerful unelected players in Ontario politics.

"Was I involved in the decision? Can you be more specific?"

Guy raised eyebrows when he testified he did not talk with McGuinty about cancelling the Oakville gas plant, and only discussed the Mississauga project with him after the then premier had made his decision to cancel that plant too.

"You are seen as a very capable and skilled campaign manager," said NDP energy critic Peter Tabuns. "You're not credible as an errand boy."

Guy admitted the Liberals didn't know how much the cancellations would cost, but said the opposition parties made the same commitment to scrap the gas plants also without knowing what the final bill for taxpayers would be.

"There was a sense that outright cancellation -- which was (the PC's) policy -- would be a much more expensive, and worse value for the taxpayer, than the relocation policy that (McGuinty) ultimately decided on, and that we announced in the campaign," said Guy.

"So you cancelled it with absolutely no knowledge or concern of the cost," said Fedeli. "It's absolutely no wonder now why Liberals continue to raise taxes."

The opposition parties were fuming after the Liberal chair of the committee ruled they couldn't ask Guy about emails showing he and other Liberals tried to pressure the Speaker into changing a preliminary finding of contempt against the government.

"Speaker needs to follow up on his prima facie finding and change his mind," Guy wrote last September in an email to senior Liberals including Brendan McGuinty, the premier's brother and a key part of his campaign team.

The Speaker's preliminary contempt ruling came after the Liberals failed to release all documents on the gas plants as requested by a committee, and was one of the reasons McGuinty cited when he prorogued the legislature and resigned as premier.

The Tories and NDP said they have a right to ask about all documents the committee was given, and noted Premier Kathleen Wynne promised to be open about the gas plants and to allow the committee members to ask any question they want.

"We want to have the ability to ask questions related to all documents that this committee has received," said Bisson. "Otherwise, essentially, the premier -- in my view -- is not living up to her word of being transparent and allowing us to be able to do what we have to do in this committee."

Bisson managed to get a barred question in anyway, and Guy responded "no," he had not been involved in any discussions about trying to influence Speaker Dave Levac to change his ruling.

"The previous question was out of order but executed with such finesse that it slipped by," responded chair Shafiq Qaadri.

The opposition parties say the Liberals spent at least $585 million to cancel the gas plants in order to save Liberal seats in the 2011 election.

The auditor general is scheduled to release a report into the actual cost of cancelling the Oakville gas plant at the end of this month, which most observers predict will send the total bill soaring much higher.

The government has said scrapping the Oakville project cost about $40 million, but the Ontario Power Authority puts the figure at closer to $310 million.

The auditor's report on the Mississauga gas plant concluded halting the project in mid construction cost $275 million, far above the $190 million that the Liberals had been claiming.