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My rule of thumb is about 100km from the outskirts if you want all the skies dark (and obviously not close to a regional centre of population).
But if you can live with glow on one side of the sky, you can get closer. Eg. if you go as far as Gembrook from SE Melbourne, you'll find all but the western sky is usable - try Gembrook Sports Ground. I found that to be reasonable some years ago - not sure about now.
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