The requests came from a couple of fellow (senior) arbiters, and during the discussion we agreed that there is a pretty small pool of arbiters in Australia to call upon. This partly explains why the same faces keep popping up at the 'big' events, although organiser preferences also play a part.
While there may be a few reasons why there aren't more arbiters in Australia, I would like to focus on one reason. I believe the lack of arbiter accreditation in Australia has a lot to do with the problem. Without accreditation there is no impetus for arbiter training, and there is no way that organisers know who is qualified to run their tournaments (apart from word of mouth). More importantly, the lack of accreditation also leads to a situation where potential arbiters are left behind/ overlooked as they themselves don't have a yard stick to measure themselves against. There may be a number of people who could do a good job of being an arbiter, they just don't realise it.
Of course the sensible solution is to have an arbiter training and certification program, as many other countries have, but movement in this area has been somewhat glacial. For the moment the training seems to be left with local clubs, with a belief that arbiters will just bubble up to the higher levels. From my experience this hasn't really happened, and therefore a new approach is what is required.