Further evidence - if any more were needed - of the weakness in Perth's property prices has been produced today, and at least one expert says it could get even worse.

The RP Data-Rismark house value index shows the city has clearly done worse than any other state capital in September.

The median price for all dwellings of $460,000 was down 1.9 per cent in August and 5 per cent in the past three months.

In contrast, Sydney prices rose 0.8 per cent in the quarter, Melbourne went up 0.7 per cent, while Brisbane fell 1.4 per cent.

Perth house prices dropped 1 per cent in the month and 4.3 per cent for the quarter, to $480,000, while units plummeted 4.1 per cent in the month, and 6.8 per cent in the quarter, to $405,000.

Rental yields increased marginally in the month for houses, to 4 per cent, while they remained at 4.4 per cent for units.

Nationally, more expensive suburbs did better, with a 0.4 per cent increase in prices against a 0.4 per cent fall for the bottom end of the market.

It is the third of the three major property indices for the Perth market recently, and while they all use different methodologies and get different results, they all show the same trend.

Fairfax-owned Australian Property Monitors reported a 1.5 per cent drop in house prices, and 0.8 per cent fall in units in the September quarter, while the Real Estate Institute of WA median house price for the three months fell 4 per cent, to $480,000.

REIWA figures also show more than 16,000 properties for sale, about 20 per cent above the "equilibrium" level, with sellers discounting asking prices by an average 6 per cent.

Prices could fall even more, Rismark chief executive Christopher Joye said.

"Home loan rates will eventually start increasing with the prospect that the peak mortgage rate could converge to close to 9 per cent (compared with 7.4 per cent at present)," he said.

"Household balance-sheets will be supported by a strong labour market and robust income growth. But let‟s be clear. It is now just a matter of time before the RBA and-or the banks raise rates again."

RP Data research analyst Cameron Kusher, echoing the words of APM's Yvonne Chan, said with rental yields increasing it was an ideal time for investors to enter the market.

"Early signs suggest that rental rates are once again improving, listings are at above average levels, and leading indicators such as time on market and vendor discounting are creeping up," he said.

"For those active in the market there is increasing scope for price negotiation and less competition amongst buyers with an above average number of properties for sale. These conditions are likely to afford opportunities to purchase property at more competitive prices."

Meanwhile, figures from the Housing Industry Association released today show new home sales fell 1.8 per cent in September.