With an election on the way let’s indulge in a not-so-hypothetical situation: you are a voter in a seat that will, eventually, fall to one of the major two parties. However you’re not particularly enamoured of either, but will choose one to be the lesser of two evils.
In the sort of seat we’re talking about here, preferential voting means that your vote will end with whichever major you put highest on the ballot paper. You get that choice regardless of who your first preference is.
It turns out that there is a very compelling reason to not give your first preference to whichever major party you deem to be the lesser evil.
To quote Tom Lehrer: I refer of course to money.
Buried in the Commonwealth Electoral Act is this:
294 General entitlement to funds
(1) Subject to this Division, $1.50 is payable for each first preference vote given for a candidate in a House of Representatives election.
(2) Subject to this Division, $1.50 is payable for each first preference vote given for a candidate or group in a Senate election.
(4) A reference in this section to a first preference vote shall be read as not including a reference to a vote that has been rejected as informal in the poll concerned.
This is why putting your first preference somewhere else is not a waste. You still get to choose which major party ends up with the seat, but you also get to:
- Deny funding to the major parties; and
- Give funding to your first preference if at least 4% of the electorate vote the same way.
The latter will make it easier for your preferred candidate/party to campaign next time around, and enough diversion from the majors might cause an upset, or send a message that the electorate are unhappy.
However note 294 (4) quoted above: electoral funding only applies to formal votes.
So choose wisely, and vote formally.
 In other words approximately 90% of electors, give or take.
 An unknown, but not insignificant, percentage in my opinion.
 See s297 which sets the 4% floor for funding.