Would recommend staying a week down south around Busselton or Margaret River if possible. It will be less hot, but still hot enough to enjoy the beaches, and there's so much to see down there: breweries/wineries, forests, caves, and heaps of beautiful coastline.
Also check out the Perth beaches, Perth hills, spend some time around Fremantle. If you're staying for a month and you're near a train line you might want to get a SmartRider. Lmk if u have any other questions!