Ruadh, you’re right to some extent. What happens is that if someone is elected on the first count, all their second preferences are counted and the ratio is applied to the actual votes transferred. So hibernicus’ concern about the order the boxes are opened in doesn’t come into play. But say no-one is elected on the first count and candidates are eliminated on that and subsequent counts with their votes transferred. In that case, when a candidate eventually does reach the quota, it’s the last votes they received on transfer that transfer on as their surplus. This applies to all candidates not elected on the first count. As you can see, it’s not really random and you always have to look at the previous eliminated candidate when trying to assess where transfers will go.
So, if you want to be cute about it, you can give your first preference to a no-hoper in the knowledge they will be eliminated, thereby placing your vote on top of the bundle of your second choice and maximising the chances of your vote transferring as candidates are elected on subsequent counts. In that scenario, you have the most influence on keeping your favoured candidates in the race as long as possible.
Anyway, this is all academic as electronic voting will do away with the fun and games. In defence of the system, think about how hard it would be to apply full proportionality with a manual count. Also, as I said, it’s not random, there is a procedure (however flawed) and it’s all done in front of the party faithful who make sure that the procedure is followed - fraud is not really a big problem.