Thanks for asking, Geoff. To best answer your question we’ll begin with a quick refresher on Senate elections.
To complete a Senate ballot paper, voters need to either number at least 6 boxes above the line or number at least 12 boxes below the line. To be elected, candidates need to gain a quota—a set proportion of the formal votes.
If a candidate receives more than the required quota, these 'extra' votes are then distributed to the second choice on the ballot paper. These ballot papers are distributed to the remaining candidates, based on preferences and the votes are recounted. This process repeats then until all vacancies in the Senate are filled.
For some voters, this may result in their vote being considered 'exhausted'. This is when all of their preferred candidates have been excluded and there are no preferences remaining on their ballot. When an 'exhausted vote' occurs the ballot paper is set aside.
More detailed information on how the Senate result is determined is available on the Australian Electoral Commission website.