The 8.9 per cent decline in activity is the largest annual drop since April 2011, CREA said, and was led by declines in major markets such as Vancouver, Toronto, Calgary, Edmonton and Ottawa.
On a monthly basis, sales were down by 5.8 per cent from July's level, the largest drop in two years.
While the number of sales declined, prices held up. CREA says the average price of a Canadian home sold in August was $350,192, up 0.3 per cent from where it was a year ago.
August offered the second month of data since the federal government moved to cool down the housing market, chiefly by returning the maximum possible amortizaion period to 25 years.
"A few more months of data are needed to gauge the broader impact of recent regulatory changes on Canada's housing market," CREA's chief economist Gregory Klump said.
"August's sales figures will no doubt provide comfort to policymakers, providing the first clear indication that the recent changes to mortgage regulations aimed at cooling the market are working as intended."
As it has several times in the past year, CREA warned that the national average price is being skewed by a several factors, most notably fewer sales in Vancouver this year compared with 2011.
If Vancouver activity is stripped out of the equation, the national average price has increased 3.3 per cent in the year up to the end of August.
"The recent slump in home sales suggests that a housing correction is underway," Capital Economics' David Madani said in a note today after the release of the data. "Assuming that sales continue to trend lower over the remainder of this year, then the typical lag relationship between sales and prices indicates that house prices will eventually follow suit early next year"
In a separate release, the real estate group updated its housing forecast for the year. CREA now expects fewer home sales this year and next, and it predicts the average home price for 2012 as a whole will come in at $365,000, a 0.6 per cent rise from 2011's average.
For 2013, CREA predicts a slight pullback, with prices ticking down 0.1 per cent to $364,500.