okay, firstly, when we're talking about "high speed" internet connections; what people are used to being high speed in other countries simply doesn't exist in Australia at all.
Don't get me wrong, we have broadband, at reasonable speeds, but there isn't yet a truly world-class competitive high-speed network in oz. (yet).
the sheer distances and sparse population are a big part of that, but also the stupid politicians play a part. (one former minister was quoted as saying something like (paraphrasing from memory) "australia doesn't need to upgrade to faster internet, because the only people who use that kind of bandwidth are criminals and pornographers"
anyway, you can easily find what places have high-speed connections by internet search. there's a LOT of competition at the moment.
most providers have somewhere on their website a page where you can put the number of your landline in and they tell you if broadband is available on that interchange. (to approximate, search for a phone number in whitepages.com.au)
there's also a great website called Whirlpool (google it, I'm not sure if its a .net or .au or what) that has huge amounts of info re ISPs and stuff, as well as a great techie forum.
anyway, Mullumbimby does have high-speed broadband