Our elections use the single transferable vote (STV) system. A good explanation of this system is available here.
STV means that if you only vote for one candidate, your vote will expire after the first round.
To win an election, a candidate needs 50% +1 of the votes.
After each round of results, if no one has more than 50%, the candidate with the fewest votes is eliminated. Voters who put this candidate as their first preference will have their second preference counted in the next round.