They won't come up through the toilet, but there are plenty of drainage holes in a house. Drains generally go into sumps in the UK, but not sure about Australia. Regardless, as properties get older, drains get lose or cracked and it does provide access. But you are right that they can get in through any crack. But they don't like being exposed, so unlikely they will come through cracks that are open to the air and visible.
Plus, they can swim, but I don't believe they can duck dive. Hence the U bend should be safe.