Col, from some of the comments you've made, I suspect your name may well appear in my log book.
Back on to the subject in hand.
The killer issue overriding all else in these purchases that go wrong (or, given how many there have been, should that be "these-purchases-that-go-wrong"?) is one that Bushranger 71 has touched on already. The Department of Defence has ceased being about providing the best and most suited equipment for our soldiers, sailors and airmen to go into battle with - (or perhaps more importantly, to be able to project Australia's interests quickly and effectively and therefore not to have to go into battle with). It has instead become, to an overwhelming degree, a Department of Defence Industry, with its main aim being to provide the biggest returns to the people who will build said equipment.
Like many such skewed ideas, it started out with noble (or as noble as things can get in the halls of government) ideas of providing the maximum number of jobs for Australian workers and developing local industry. However, as defence equipment has become more complex and so expensive to buy, local defence companies have found it impossible to bid as stand alone contractors, and so very few have survived as independant entities. Instead, almost all have become branches of major European or US defence conglomorates and the local jobs such purchases generate are all too frequently short term or so so uneconomic in terms of what the taxpayer has to fork out to generate those jobs, it's just not worth it. And, most importantly, much if not most of the profits now go overseas.
Insisting on a specialised local product, (which involves huge added expenditure, as well as very long delays), has also been been shown - in almost every case - to be a disaster, and after huge outlays far in excess of what originally budgeted for, in some cases, (Seasprite), resulting in no product reaching the end user at all, while in others, (Tiger), political pressure has resulted in the end user being forced to take delivery of a product that clearly was not reaching many of the promised performance parameters set down in the original contract and promised by the manufacturer. This too has resulted, to date at least, in no product reaching the end user, and the end user having to accept that when the (incredibly expensive) product eventually arrives, many years after it was due, it will not perform as promised.
Added to this is the very ignoble aspect of some people at the top, both the civilian 'suits' - (and I stress that I include very senior politicians among these 'suits' - just look who some ex-ministers of the Howard government are working for now) -and those in uniform who have allowed the job they have been promised post-retirement, or the job they hope to win post-retirement, to colour their judgement in what company should be chosen for a particular contract. (This is by no means peculiar to Defence.)
In short, a very unhappy situation, and one that, if the Australian population was not so apathetic, (or perhaps just plain ignorant) about how much has been spent - and continues to be spent - to so little effect, would be causing an outcry as loud as is currently being made over the ill-conceived carbon tax.
In an attempt to bet back onto the topic of this thread, the C-17 purchase has shown how any such purchase should be done - and for this country to be running a fleet of King Airs in lieu of a Caribou (or its modern day equivalent) fleet is a travesty of the first order and one that will cost us dearly should we ever have to go into battle in our local area.