TORONTO - Premier Dalton McGuinty must close the legal loophole his government exploited to rush partisan ads promoting its controversial tax harmonization plan out the door last spring, opposition critics demanded Tuesday.

He should also repay taxpayers the $700,000 spent to sneak in the online ads, said Conservative Leader Tim Hudak.

"The auditor ruled that these ads were partisan in nature -- they're breaking Dalton McGuinty's own legislation," he said.

"If Dalton McGuinty truly is a man of his word, the Liberal party should pay that $700,000."

The premier denied the ads were pushed through the back door by putting them on the Internet, the only catch in the law his government put in place to stop partisan advertising.

"We're doing what we can to get the word out," he said.

Documents released last week showed the Ministry of Finance paid $600,000 to buy space for online ads promoting the HST and another $95,000 for Narrative Advocacy Media to come up with the campaign.

The Liberals were prepared to spend about $2.5 million on the campaign, which was to include newspaper and radio spots in French, English, aboriginal and ethnic media. But only the online ads were used because the the print ads were rejected by auditor general Jim McCarter for being too partisan.

Under provincial law, the auditor must approve all print, television and radio ads paid for by the government. McCarter has no say over ads that appear on the Internet.

The Liberals put the legislation in place several years ago to curb what they described as abuse of taxpayers' money by the previous Conservative government.

Now the Liberals are skirting their own rules to get the ads out, said NDP Leader Andrea Horwath

"Everybody is saying these things are wrong, they shouldn't be using taxpayers dollars to bring partisan messages to the people of this province," she said.

"That's what this is all about and it just shows the same character that we've seen from this government for quite some time."

Revenue Minister John Wilkinson, who inherited the HST file in June from the Finance Ministry, wouldn't say whether the government planned to close the loophole or pay back the $700,000.

"All I know is that as the minister of revenue I will continue to do what I've always said: make sure that any ads that are approved by the Ministry of Revenue have the stamp from the auditor general, who has had a chance to review all of that," he said.

Wilkinson said he met with McCarter to review his concerns about the HST ads, and the two found a way to work together to get a second HST campaign off the ground last week.

"We take a lot education from the process," he said.

The latest HST ads, which appear in print and online, were approved by the auditor and will cost $1.6 million.

The ads highlight an income tax cut that took effect Jan. 1 and invite readers to "take a closer look at Ontario's new tax package" by visiting a government website. Wilkinson said he wasn't aware of any other HST ads put out by his department.

The current campaign, which runs for three more weeks, will be monitored closely to see if it drives more traffic to the website before any decision is made on whether it will be expanded or extended, Wilkinson said.

The opposition parties are bitterly opposed to merging the federal GST with provincial sales tax, saying it will kick people who are already struggling during the economic downturn.

Tax harmonization will increase the cost of many items that were previously exempt from the provincial levy, from gasoline to Internet bills, haircuts and real estate fees.

Like Ontario, British Columbia will also harmonize its PST with the GST on July 1, something Quebec, New Brunswick, Nova Scotia and Newfoundland and Labrador have already done.

Federal Finance Minister Jim Flaherty, who has long advocated for tax harmonization, agreed to kick in $4.3 billion to Ontario and $1.6 billion to B.C. in a deal struck last year.

Much of the federal cash going to Ontario will be used to fund government cheques of up to $1,000 for families and $300 for individuals in the first year of the HST to offset the pain.