Progressive Conservative Leader Tim Hudak is promising a judicial inquiry into the contentious cancellation of two Toronto-area gas plants, going after the Liberals on a scandal that dominated the last provincial election three years ago.

Hudak's pledge -- which he has made in months past -- was reiterated at the site of one of the cancelled plants in Mississauga, Ont., where unfinished concrete pillars and a tangle rusting of metal rods could be seen behind him.

"A billion dollars went into this hole," he said. "This was a deliberate decision knowing the costs of the cancellation. Knowing it was going to cost us more, doing so just to save a Liberal seat during an election campaign. That's wrong."

The Liberals scrapped the unpopular gas-fired power plant in Mississauga during the 2011 election and in neighbouring Oakville in October 2010, at a cost of what is the auditor general has said could be up to $1.1 billion.

Construction on the Mississauga plant continued for weeks after the last election, but the decision to cancel it helped save five Liberal seats in the area.

A legislative committee had been probing the cancellations until the current election campaign began, and there is also an ongoing police investigation into the deletion of emails and documents about the unpopular plants, but that's not enough, said Hudak.

"We have to send a very strong signal to governments of any political stripe, that this kind of behaviour just won't be tolerated in the province of Ontario," he said.

"If we want to give investors confidence to invest, if we want to give taxpayers some confidence that they're not going to see this abuse, we need a judicial inquiry."

Premier Kathleen Wynne has apologized for the way the gas plants issue was handled and has said her predecessor Dalton McGuinty, under whose watch the plants were scrapped, made "mistakes."

But Hudak scoffed at that defence.

"A mistake is like when you get a parking ticket. This was not a mistake, this was a deliberate abuse of your tax dollars to save Liberal seats," he said. "We need to draw a line and say this is not going to happen again."

The Liberals have pointed out that Hudak said during the last election campaign that he would scrap the Mississauga plant if he formed the next government.

They also took a jab at the Tories' energy policies on Sunday, saying Hudak has failed to learn from their cancellation decisions and alleged a Progressive Conservative government would rack up millions in costs by scrapping certain renewable energy projects.

"He wants to cancel renewable energy contracts," said Liberal MPP Brad Duguid. "What that's going to do is cost potentially tens of billions of dollars, 20 times more than the gas plants cost us."

The Tories have pledged to replace subsidies for wind and solar power, which they estimate will save $20 billion a year, and invest in nuclear, natural gas and hydro power.

They've said it would be up to the energy minister to decide whether to proceed with large scale wind and solar contracts that have been approved but aren't yet producing energy, but any project that is cancelled would end under existing contract termination clauses.

Meanwhile, the New Democrats repeated their call for a public inquiry on the gas plants on Sunday, saying it's something they've long been committed to seeing through.

"Frankly we saw Liberals over the last number of months and year not being upfront and honest with Ontarians about who knew what and when," NDP Leader Andrea Horwath said in Toronto.

"I still believe there are answers that people need in terms of the coverup as well as the gas plant cancellation decision itself."

Ontarians cast their ballots on June 12.