With gas prices up about eight cents per litre as a result of the HST taking effect, NDP Leader Andrea Horwath used the opportunity to say an NDP government might try to do something about the tax in five years.

"We know the government has basically tied our hands for five years," she told a Queen's Park news conference on Thursday, a day that saw gasoline cost about 104.4 cents per litre in the GTA.

The Liberal provincial government signed a $4.3-billion deal with the federal Conservatives that provides payments to ease the transition into the new tax, which harmonizes the old eight per cent Retail Sales Tax with the five per cent GST.

"We're not going to say we can change things at the snap of a finger," Horwath said.

"For example, in Nova Scotia, they ran on a campaign that took the HST off of home heating costs."

However, in Nova Scotia, which has an NDP government, the HST rate is set at 15 per cent -- two percentage points higher than Ontario.

British Columbia, governed provincially by the Liberals, has an HST of 12 per cent. It doesn't apply the HST to gasoline or diesel purchases. But it doesn't offer the transitional rebate cheques that Ontario has paid out.

The government of Ontario's Premier Dalton McGuinty has argued the HST will remove red tape from businesses and help create 600,000 jobs over 10 years.

It argues the HST helps business because it is a value-added tax, meaning a business only pays tax on the value it creates, not on its inputs. Businesses that operate in jurisdictions that have HSTs have a cost advantage over Ontario businesses, it has said.

The government has argued that prices for consumers will come down as businesses start passing on their reduced costs. In addition, Ontario businesses' exports will be more competitive in the global economy.

"It shifts the tax burden off of corporations and onto ordinary, every-day people and small businesses," Horwath said. "We think that's wrong. We don't think that's fair."

She promised to continue making the HST an issue until the October 2011 provincial election.

Ontario's Revenue Minister John Wilkinson told CTV News Channel on Thursday that the HST will only drive prices up for about 17 per cent of Ontarians' purchases.

"We're taking of that (HST) money and we're lowering income taxes for people -- some $12 billion over the next three years," he said.

"It's all about strengthening our economy and getting people back to work."

In his attack on the HST on Wednesday, Ontario Progressive Conservative Leader Tim Hudak didn't commit to either repealing the HST or cutting the rate. He would only say all options are on the table.

Hanging over all government economic decisions is a budget deficit of $21.3 billion for this budget year.