There is 1 satellite signal transmitted from the studio in Canberra, and that is the North west version of GWN. It is then downlinked in Bunbury, and using remotely controlled equipment in the Bunbury studio's, they insert south west ads and then send it on it's way to the Mt Lennard transmitter.
The way that Margaret River works is that it just takes an 'off-air' feed (anotherwords, what you see at home if you lived in Bunbury with a normal TV antenna on your roof) and re-transmits it to everyone in Margs.
The reason being is that both the Mt Lennard and Margaret river transmitters are high enough to be able to recieve signals from each other, but not everybody's TV antennas at their house are able to do that
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The great southern ads are also inserted from the Bunbury studios the same way as the south west ads.
It's all remotely controlled from Canberra, which is quite scary in a way, but I believe it works fairly well for them. They had quite a few hiccups in the first 12 months of switching over though, that's for sure.