SYDNEY: Can get very hot days, especially if you're living in the western suburbs. You'll have around, on average, 5-8 days over 40C during the summer. Thankfully, we are saved by southerly busters in the afternoon that bring rain, cool winds and thunder. Even though the summers can get awfully dry, they can also be very rainy and humid/sticky (in fact, most of our rain falls in summer). Winters are cool to mild. Rarely the maximum (daytime) temps fall to 15C (maybe once to three times in the winter). The temps are always around 17-21 in the winter. Winter is also very sunny and dry.
PERTH: Even hotter than Sydney. This city has the classical Mediterranean climate with hot, dry summers and cool, wet winters. Summers have a lot of days with the temp being around 35-40C. Rainfall is almost unknown in summer. Winters are also slightly warmer than Sydney's. Temps are usually around 19-23. Though, they have some rainy days.
ADELAIDE: Australia's driest capital city. Like Perth, this has a Mediterranean climate, but it's much drier. Adelaide also has a lot of days over 35C and they reach 40C. Winters, however, are cooler than Sydney's. The temp mostly averages at 14-17 degrees. Winters can be a little wet, too.
MELBOURNE: This city has the strangest climate in Australia, if not the world. One day it could be scorching hot and the next freezing cold. It could be also raining in one hour and in the next it could be sunny. Summers can get extremely hot, yet they can also have days with a cold spell. Weird climate! Winters are cold and a little wet. You can have days around 12C, but they usually linger around 14-16.
BRISBANE: Subtropical and mostly humid. Summers are always warm (not hot, really) with some cloudy days. The temp during the summer is around 30-34C. It never gets any lower or higher than that range. Winters are mostly around 21-25 (quite mild). Though you may have some days with the temp around 18-21. Brisbane also has a lot of rainy days, since it's subtropical.