Perth house prices have fallen more than four per cent in the past three months as the resources capital continues to feel the effects of the mining downturn.

The median house price in the West Australian capital fell 4.2 per cent to $522,133 in the September quarter, down on the revised June quarter median of $545,000, preliminary figures released by the Real Estate Institute of Western Australia show.

Unit prices slid 2.3 per cent to $420,125 during the same period.

REIWA President Hayden Groves said stock levels were high, with the number of properties for sale in Perth hitting 16,000 in the past week.

Housing stock on the market was now at levels not seen since the immediate post global financial crisis days, and well above the long term average of 12,000 properties, he said.

It takes an average of 62 days to sell a property in Perth, up five days on the previous quarter, and more than half of vendors have been forced to discount their asking price in order to sell, Mr Groves said.