As others have said, some of the transmitters for the air band frequencies are located at the airports themselves, but others are off-site, located up in the Darling Range for better coverage.
For example, this is one of the Melbourne Centre frequencies - 133.900MHz:
From the information on that page, we can see that the site in the hills is transmitting an AM voice signal with 6kHz bandwidth at 50 Watts of power on 133.900MHz.
By comparison, the tower frequency for Perth Airport's record shows:
In this image, we can see that the site is the control tower at the airport, the signal is AM voice again with a 6kHz bandwidth, same as the other one, but this time the signal is only at 20 Watts of power on 120.500MHz.
This means the transmissions from the lower are at a lower power level, and the transmitter is located down on the flat land of the plain, rather than up in the hills. Which is why SOL will be having trouble picking up these types of transmissions from Joonadlup...
Hope this helps
By the way ... a couple of links ...
This link lets you search for frequencies and sites as I did, while
this link shows what "Melbourne Center" actually looks like
![Wink ;)](./images/smilies/icon_wink.gif)