TORONTO - A loophole in Ontario's law requiring disclosure of all public sector salaries over $100,000 may have allowed hundreds of people to hide their true pay levels, the opposition parties said Friday.

The Public Sector Salary Disclosure Act was introduced by the Mike Harris Conservative government in 1996, but it turns out per diem income reported on T4-A slips was not being disclosed by everyone.

"I am absolutely concerned because I would bet almost anything that there are dozens, maybe hundreds of people who are being shielded and hidden from the sunshine list," said NDP finance critic Michael Prue.

The Liberal government vowed to move quickly to tighten the rules around salary disclosure to include per diems, and said it did not expect "a significant number" of public servants failed to disclose how much they were paid.

The Progressive Conservatives complained Ontario's public sector has grown "big time" under the Liberals and said the government should have closed the salary disclosure loophole sooner.

"It's something we support and it's something that should have been done long ago," said Opposition critic Monte McNaughton.

"Dalton McGuinty has completely ignored this, and it's his approach to government and to this whole public sector bloating. These people getting paid on a per diem basis should be on the sunshine list."

The problem came to light after McGuinty recently nominated Progressive Conservative Elizabeth Witmer as chair of the Workplace Safety and Insurance Board at a salary of $188,000.

The outgoing chairman, Steve Mahoney, made $129,000 last year, but he wasn't on the sunshine list.

The WSIB said his income wasn't disclosed because Mahoney, who was paid $550 a day, was a part-time chairman. The position has been made a full-time job for Witmer, who will not be paid a per diem on top of her salary.

The government came under fire when the 2011 list was released in March because it did not include the sky-high salaries of former top executives at the troubled Ornge air ambulance service, including the $1.4 million paid to ousted chief executive Chris Mazza.

"We know that Mazza was hidden from the sunshine list, we know people at Ornge were hidden (and) we now find out that Steve Mahoney was hidden," said Prue. "There's going to be a lot of them."

There were 78,910 names on the sunshine list for 2011, up 10 per cent or 7,432 workers, from 2010. The NDP predicted that number will grow even higher once all public sector income is reported.

"I for one think that when they get the rules in order and do it right, you're going to see a whole lot more there than what's been there in the past," said Prue.

Not everyone paid by taxpayers on a per diem basis failed to report on the sunshine list. Janet Robinson, who was paid $627 dollars a day as the part-time chair of the Ontario Film Review Board, reported she made just under $103,000 in 2011.

The Liberals say it was the Tories who left income reported on T4-A slips out of the original 1996 legislation.

"The way the law was written encapsulates T4s only. We'll have to figure out how to amend the law so that it also captures T4-A's," said Finance Ministry spokesman Darcy McNeill.