We also have the strictest party discipline in the world, so there's very little room for back-scratching.
Finally, our Constitution bans any bill about appropriations from dealing with any other subject. So you can't tack riders onto other subjects.
That's a really clever appropriations bit in your Constitution. I've often thought that the US needs some sort of Kevin Bacon Law -- all the measures within a bill have to be related to each other with only a few degrees of separation. No more banning internet poker in a port security bill! (https://en.wikipedia.org/wiki/SAFE_Port_Act)
95% of Australians will defer to their parties' chosen preferences, rather than stating their preferences explicitly. This is mostly because you have to number hundreds of boxes if you chose to vote "below the line".
Some argue that parties' preferences will not always reflect the intention of the voter - but they get away with it because its too difficult to vote preferentially.
I also hear that in practice, the third parties are basically bribed with minor legislative concessions if they form a coalition with a major party and cast the deciding vote in favor of their legislation.
Still better than the third party not existing & having a say at all, though.