Flooding of the ocean - like you might get in low lying countries in Europe - is rare or non-existent here. Though there are the doomsayers who have been predicting massive tsunamis on the East Coast sometime soon. But I doubt very much there is any real science behind that.
I'm sorry to have to tell you that it's not only rivers that flood though. Sometimes a little creek can cause significant flooding too. And you can still get flash flooding without a river or creek anywhere near. I believe it can happen anywhere in Australia, but some parts are indeed more prone to flooding than others.
The QLD floods were really extreme, even to Australian standards. Most of the time flooding is merely an annoyance, not a life threatening event.
Australia really is a country of extremes. If it isn't floods, it is drought and bushfires. But you get used to that idea. Just like you get used to the idea of deadly spiders and snakes around your house.
We had the neighbouring town flood last year after 7 years of severe drought! Suddenly the heavens opened, the dams filled up to the point where they had to let water out, which swelled the river, which flooded the town downstream (awful neighbours we are, I reckon!). From one extreme to the other...