Doing it by numbers

When I was a kid, Brisbane phone numbers had six digits, then they had seven, and now they have eight. I'm no maths whiz, but I don't quite get why they have to be that long - and be made even longer with the addition of state codes. Surely eight digits provides more than enough combinations of numbers for the whole of Australia, let alone just Queensland - especially since fixed line connections are waning in favour of mobiles.