In my twisted logic it is sometimes better to vote for the worst of a bad bunch. More specifically, vote for the incumbent repeatedly, which will (hopefully) force the (primary) opposition to eventually change to a more real alternative. Either this, or they (the opposition) will become indistinguishable from the incumbent (then they could merge or form a coalition). In this case, the smaller party/ies will (hopefully, eventually) build up a large enough vote share to form a real opposition. It certainly would seem strange to see the Labor and Liberal parties merge/form a coalition, but it really doesn't seem too far away.
I've often thought that having similar parties might actually be a very good thing in (or, a good sign of) a stable democracy. Kind of like the way when you've got one of those jelly-bean-in-a-jar or the-price-is-right guessing competitions and the best strategy is to pick a number exactly one more or one less than the other person (as much as it may irritate them).
If things are working properly, then compromise/steady-state should be almost fully resolved before you get to vote - when things are stable, voting is then there to apply the pressure on parties to reach that compromise/steady-state.
Anyway, I'm not a political scientist by any means... so please point out the errors in my logic!
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