Ontario property owners will see a double-digit increase on their assessments because of the hot housing market and because the province's property assessment freeze has lifted, an expert warns.

Maureen O'Neill, president of the Toronto Real Estate Board, says the median price of a detached home in the GTA has increased about 20 per cent since the last assessments were done in 2005.

Three years ago, the median price of a detached home in the GTA was $335,000. Today, it's $403,000, O'Neill said.

Condominium and apartment owners will also see a whopping 27 per cent increase in their property assessments, she said.

In 2005, the median price of a condo or apartment was $192,000. Today, it's $245,000, O'Neill said.

"They've seen appreciation and therefore they're going be taxed accordingly," she told CTV Toronto.

"(Property owners) are going to experience quite a dramatic shock."

Homes in east Toronto will be particularly hard hit, as assessments will rise about 31 per cent, O'Neill said.

The province's three-year freeze on assessments lifted on Jan. 1. The Ontario government, however, has introduced a plan to spread out any assessment increases over the next four years.

NDP Leader Howard Hampton, however, said the government's strategy, including the freeze, has made the housing assessment ordeal worse.

"There's a big problem here. The property tax is not based upon someone's income, it's not based on my ability to pay," Hampton said.

The Municipal Property Assessment Corporation came under fire years ago for conducting inaccurate assessments. The agency performed 500,000 on-site inspections last year in its bid to boost accuracy.

Property owners will be receiving their assessments in August and September.

With a report from CTV Toronto's Paul Bliss