Very broad question!
My 2c worth:
If you/your girlfriend are studying astronomy and hence interested in working in the field, I'd be sticking to NSW or ACT. I don't think the other states really have much in the way of capabilities to support a career in astronomy. Sure there are institutions, observatories, etc here and there - but not like in ACT or NSW, is my impression.
That narrows your search down a little, to those states.
As for dark skies - I'm no help, I live in WA.
Dark skies everywhere here. Living in the hills the skies are average (can easily see milky way, lmc, smc) but still light polluted. There are places here you could live 1.5 hours from the CBD and have quite dark skies, but that's a bit of a drive. Anyway, I don't think you want to be looking in WA considering your interest in studying/career.