Rents fell in all eight capital cities last month, with annual rental growth of just 0.9 per cent the slowest over the past two decades.

CoreLogic RP Data's monthly rental report shows an average 0.3 per cent decline in capital city rents, with a 0.9 per cent slump in Darwin the biggest and 0.1 per cent declines in Brisbane and Adelaide the smallest.

Darwin also had the biggest annual drop in rents at 9.3 per cent, while Sydney had the largest annual rise of 2.5 per cent.

Australia's biggest capital also had the nation's highest median rent at $593 per week, with houses at $610 and units at $536.

CoreLogic RP Data's head of research Tim Lawless said the capital city average growth of just 0.9 per cent over the year to July was the slowest in records that go back to December 1995.

"Rental appreciation continues to be sluggish and can be largely attributed to the ongoing boom in dwelling construction across Australia's capital cities accompanied by record high participation in the housing market from investors," Mr Lawless noted in the report.

"In particular, a high proportion of the inner-city unit development is being targeted by domestic investors and foreign purchasers."

Smaller markets lead auction clearance rates higher

While the last month saw rents fall across every capital city, the property purchase market has not shown immediate signs of slowing down.

The most recent weekend's auction clearance rates rose slightly to 77 per cent nationally, from 74.6 per cent the week before, according to CoreLogic RP Data.

That is despite more properties being auctioned last weekend than at any time over the previous nine weekends.

Another interesting aspect of the most recent figures is the jump in auction clearance rates in the smaller markets of Adelaide, Canberra and Tasmania, which were all above 80 per cent.

The boom markets of Sydney and Melbourne were both just below 80 per cent clearance rates, with Melbourne slightly higher following a recent dip in successful auctions in Sydney.

Although both Sydney and Melbourne had still higher clearance rates than at the same time last year.

Perth's auction clearance rate of just 44.4 per cent reflects the weakness in its rental markets, showing a broad-based real estate decline in that city.

For the bulk of properties that are sold by private treaty rather than auction, CoreLogic RP Data's figures show that Sydney's median house price is still well below the $1,000,000 mark, at $870,000, while Darwin was the next highest at $594,000.