Ontario Premier Dalton McGuinty says he won't close a controversial advertising loophole. (Canadian Press)

The New Democrats are asking Ontario's information and privacy commissioner to investigate whether there was political interference in a freedom of information request about the government's HST ads.

NDP Leader Andrea Horwath says her party requested documents about the ad campaign last summer, but were told by the government that they didn't exist.

The documents turned up in a pile of government papers distributed to reporters last Thursday, the same day they were expected to be released through freedom of information requests made by opposition parties.

Horwath says she suspects the documents were deliberately withheld and is asking the commissioner to find out why.

Finance Minister Dwight Duncan, whose ministry was in charge of the ad campaign, was not immediately available for comment.

The documents also revealed the governing Liberals spent about $700,000 to promote the HST last spring, including ads that weren't approved by the auditor general.

Only the online ads were used because the print ones were rejected by the auditor for being too partisan.

In Ontario, the auditor must approve all print, radio and television ads paid for by the government, but has no say over ads that appear on the Internet.

Premier Dalton McGuinty said Wednesday that he won't close the loophole in his own law to stop partisan advertising.

It's important for his government to have the ability to communicate with Ontario residents and does so in a "very limited way," he said Wednesday.

He said provincial advertising on the harmonized sales tax has been "relatively modest" compared to the federal Conservatives, whose budget ads are all over TV.