As Samuel Russell mentioned in his comment, Tasmania's House of Assembly is elected using the Hare-Clark variant of the Single Transferable Vote system.
The person is correct in that Hare-Clark is a form of preferential voting, and you are not required to fill out all the preferences, thus making it "optional preferential". However, in Australia, when used without further qualification the term "preferential voting" generally refers to instant runoff voting as used in the House of Representatives and most state lower houses.
Therefore, while Hare-Clark is an optional preferential voting system, saying that it's the same as "the optional preferential voting system" is wrong. Instant runoff voting is only used for single-member electorates, whereas Hare-Clark/STV is used for multi-member electorates (mathematically, IRV is the same as if STV were used for only one seat). The provision in your organisation's constitution could probably do with some redrafting to make it more specific.
Hare-Clark is very similar to the way the federal Senate works - a quota is determined based on the number of votes and the number of positions to be filled, and a candidate must reach the quota to be elected. If a candidate exceeds the quota, their surplus votes are distributed to other candidates, and if no candidate reaches quota, the lowest-ranked candidates are excluded and their votes are distributed until another person is elected.
It's quite a complex algorithm with many little details and corner cases, but once you understand how to conduct the count, it actually works very effectively for situations like the one you're describing - where the number of candidates and positions is reasonably small, and the voters know the candidates well enough to have strongly formed opinions about individuals. I'm involved in a student club at my university where we've adopted Hare-Clark for our annual elections, and while it's caused us no end of administrative problems in getting the counting done correctly, it has given us results which reflect the will of the members very accurately.