"RP data is about 275,000 total listings (need to subscribe)
SQM Research for total stock on market. ~375,000 http://www.sqmresearch.com.au/graph_stock_on_market.php?national=1&t=1
or Realestate.com 428,000 listings
(Search any suburb via map view and then zoom out to see entire country.)"
Seems there is some discrepancy in the data, which is nothing new for this industry. Maybe realestate.com is counting some that have been sold but still on the database.