I just made a comment in the "Notice to the PPT" thread - http://uselectionatlas.org/FORUM/index.php?topic=96549.msg2009906#msg2009906
I draw it to the Convention's attention in this thread as it is something that could be considered in the different models that are floated - the means by which casual vacancies may be filled for STV-elected seats. I am totally satisfied with holding fresh elections for non-STV seats, but I think another format should be used for STV elections, as a minor party may be elected which has no hope of returning a replacement candidate (and thus leaving its voters voiceless) in a single-member replacement election. Of course, it could be argued that those voters should have considered the reliablity of their candidate prior to the election, however I believe that those minor parties should not be disadvantaged in this way (that's just my opinion, however).
As I see it, there are three ways we could fill these casual vacancies:
1. As we currently do - a Special Election/By-election is held to fill the casual vacancy, in which the election is an IRV election to fill a single seat.
2. By allowing the party to which the Senator was a member to appoint the Senator's successor. This is how things are done in the Australian Senate. It ensures that the same party retains the STV seat. The problem would lie with a party such as the SDP that has subsequently disbanded (and merged with the JCP) - if the party no longer exists, how may it appoint the successor... unless the Senator changed parties also... but what if the Senator changed parties and the party that elected him or her still exists - which party should appoint the successor? Anyway, these are things that could be discussed (personally, I think if the Senator changed parties, whatever party he or she belonged to at the time of his or her resignation occurred should be the party that appoints his or her successor).
3. A countback method by which the ballots cast at the election at which the Senator was elected are re-examined, with the Senator excluded from the count and his or her preferences distributed among the remaining candidates to determine who should replace him or her. This is frequently how local councils in Australia (at least, the ones that use STV) fill casual vacancies.