Always write ‘1’ beside the candidate who best represents your interests, even if they're unlikely to get enough votes to win the seat. Getting above a certain % of the vote, means candidates get a refund on their registration fee - and it tells that party that they have support! It also sends a message to the eventual winner that the issues that candidate campaigned on are important to people in that electorate.
The candidates you list as your second, third, or more, preference will get your vote, and hopefully win! Usually before an election, you've got a good idea of the two or three candidates who are actually battling it out: probably Labor, Liberal, and a green/independent/teal if you're lucky (or some right-wing horror if you're unlucky). As long as you put the Liberals/Nationals below Labor, your vote will safely flow to help us elect the least shit candidates possible!