The NDP says the Liberal's proposed harmonized sales taxes will mean a seven per cent increase in costs for condo owners in the province. (CBC)

Ontario's plan to merge sales taxes will mean a seven per cent increase in costs for condo owners, NDP Leader Andrea Horwath said Thursday, as she urged people in the province to fight back against the changes.

Although the harmonized sales tax does not apply directly to condo fees, Horwath said, it will increase expenses such as hydro, landscaping, repairs and maintenance.

"All of these little pieces are going to be taxed eight per cent more, and so when you combine that effect, you're going to see maintenance fees overall rise by about seven per cent," said Horwath.

That would mean an increase of about $28 a month for people who pay monthly condo fees of $400.

"Governments should be coming up with policies that make housing more affordable for people, not less affordable," Horwath said.

"But unfortunately, the McGuinty government doesn't see things the same way and they've decided, through the implementation of the HST, that it's all right to make housing less affordable for people in Ontario."

Horwath is urging all Ontarians to sign her petition to stop the tax changes, but Premier Dalton McGuinty has given no sign he's willing to back away from the single 13 per cent sales tax.

The province has set up some measures to cushion the blow to consumers, such as cheques of up to $1,000, which are supported by $4.3 billion from the federal Conservative government under a deal with the province to harmonize taxes.

Yasir Naqvi, parliamentary assistant for Revenue Minister John Wilkinson, said harmonization will bring the cost for businesses down, which could in turn be passed down to consumers.

"We're trying to create a tax system which is fair, we're trying to create a tax system which is modern and which really creates jobs for Ontario moving forward," he said.

"But we are just not stopping at harmonizing the two sales taxes, we're also bringing corporate taxes down and personal income tax cuts."

There are over 350,000 condo units in Ontario.