An unseasonal, sharp rise in property listings in July may be an early sign the real estate boom centred on Sydney and Melbourne is nearing its peak.

SQM Research figures show that nationally the number of properties listed for sale rose 3.8 per cent in July compared to the previous month, and was up 4.2 per cent on the same month last year.

The biggest monthly rise amongst the capital cities came from Sydney, where listings jumped 18.5 per cent, although the number of properties for sale in Australia's largest city remains below the levels in Melbourne, Brisbane and Perth.

The number of properties listed for sale in Sydney was up 6.1 per cent on the level seen in July last year.

On a monthly basis, listings also jumped 7.8 per cent in the red hot Melbourne real estate market.

SQM Research noted that the Sydney and Melbourne rises were "abnormal for July as sale listings are generally subdued this time of the year".

The increase in properties listed for sale in the two cities is the largest recorded for the month of July since SQM's listings data began in 2008.

SQM's managing director Louis Christopher said the unseasonal, early jump in listings ahead of the traditional spring selling season may be a sign that Australia's two booming property markets are nearing their peaks.

"I believe the results do represent an indication that the measures undertaken by APRA [the banking regulator] in slowing investor demand are now having an impact on the market," he wrote in the research.

"At this stage it is too early to call the magnitude of the impact, however it is likely that the measures (which SQM still regards as moderate) will slow the rate of dwelling price inflation recorded, rather than create a price correction."

'Confident to call a slowdown'

Speaking over the phone to the ABC, Mr Christopher said he was quite sure that property price growth was now peaking.

"I'm looking to see more information come through ... but I'm confident enough to call a slowdown," he said.

"[APRA's measures have] created some caution in investors' minds that this property boom in Sydney and Melbourne may be coming to an end."

Mr Christopher said it was difficult to tell how significant the slowdown would be, or whether it may even turn into price falls at some point.

"Investors will leave the market when capital gains have peaked ... vendors are looking to cash out at the top of the market," he added.

On an annual basis, it was the struggling property markets of Darwin (34.8 per cent) and Perth (23.5 per cent) that had seen by far the biggest rise in listings.

Figures out yesterday from CoreLogic RP Data showed that those two resources-reliant centres had seen price falls over the past year, as demand fell at the same time as more owners tried to sell.