I don't think that Australia did due diligence on it's MRH, Tiger, MU90 and Seasprite acquisitions and I wonder how well structure their business cases for each project was. That's the impression I get anyway. With the Seasprite it wasn't the actual aircraft itself, but the upgrades in technology that the RAN wanted and stuck to when it was apparent that it wasn't going to work. Pig headiness maybe but bloody expensive for nothing. I think with both the MRH and Tiger the ADF should've had project teams and engineering teams embedded with both the manufacturers in Europe right from the start and also order extra airframes as spares. We basically got 1 - 2 of our NH90s for nothing because NHI had to pay for the freight out by Antonov 124 due to the delays, and that cost NHI something like $20 million per flight apparently.
Having said all that, our procurement processes then weren't exactly stunning either, but since then we've learned quite a bit and now they've become quite professional, but there's always room for improvement. One of the things that they do well now is due diligence and the procurement system is a partnership between MOD and NZDF, with MOD co-opting NZDF subject matter experts as and when needed, as well as hiring appropriately qualified civilian experts for individual projects.
The Seasprite IMO is a bit of a special case. In part because of the level of capability desired in the avionics fitout, and then AusGov adopting FAA regs for helicopter backup systems after the avionics issues had been resolved, which then required more/new work on the avionics.
It has been something like a decade, but IIRC the FAA had adopted a requirement that single-pilot helicopters using a digital flight control system had to have a backup digital flight control system in case something happened to the primary during flight.
With the desired Seasprite avionics, the RAN had requested design work to reduce the crew of RAN Seasprites from a pilot & co-pilot plus WSO to just a pilot & WSO. This was also prior to Australia adopting the FAA rule regarding backup digital systems. Therefore, Kaman got to work developing a new cockpit and avionics package so that a crew of two could operate a Seasprite, leading to the development of a digital cockpit and a primary flight control system that was also digital. However, at the time the back up flight control system was a manual one since there was not a requirement to develop a backup digital one.
A number years go by, and Kaman is just about ready to submit the helicopter based upon the requirements provided, but the Australian gov't in the mean time had adopted the FAA rule about requiring a digital backup for a digital primary flight control system. Kaman could have changed the manual back up to a digital back up for an extra AUD$24 mil. IIRC, but the stumbling block is that while the cost was not all that high, it would have required a further two years of delay as new design work and testing would have been needed. IIRC Kaman also made the argument that the manual backup system was perfectly adequate and tried to obtain a waiver for the digital back up. In the end, the decisions were that no waiver would be granted, and that Australia could not wait a further two years and instead just sold the Seasprites.
In the above, it was not so much an issue with due diligence, but more about a very ambitious programme, coupled with changing goalposts.
As I mention elsewhere (RAN thread, perhaps?) for some of the other kit, I believe the end decisions were made mostly by pollies, who made the decisions based upon what was most politically advantageous for them and/or their party, and not what was the 'best' or desired piece of kit by the services.
