Couple notes:
The import OME Dakar springs are absolutely used throughout Aus, same manufacture and spec as the stuff we get in the US. For example the Expeditions7 fleet which circled the globe, ran on OME Dakars, 7 continents, ~200k combined miles of fully-loaded travel across a variety of terrain including some of Aus's toughest outback (see Canning Stock Route), zero failures. Vehicles have run the Baja 1000 with OME products, pretty grueling test of their components and they finished (the first class champion Lexus LX570 ran some OME suspension components). Can't think of better tests than that?
While OME and TT leaf springs were all made in Aus at one time, none are to my knowledge today. I know OME, Dob, King and TT make their coil springs in Aus to this day, but neither OME or TT claim their leaf springs are from Aus. Dob does make "some" leaf applications in Aus. As for periphrial suspension components (shackles, pins, bushings, shocks, etc), OME still manufacures a great deal of their suspensions parts in Australia (shackles, pins and shocks for example) while I know TT uses an well known India manufacture for many of their shackles and bushings. Quality is all about the control and inspection process.
I don't know much about Ironmans place of origin but from what I was told by the previous IM rep and reps from the other Aussie brands, they do zero manufacturing in the Aus. I can tell you we've pulled some pretty sad IM products off of customer vehicles, such as failed caster correction bushings that were ~6 months old and let the truck handling dangerously. The sleeves had completely separated from the bushing itself.
We've been selling/installing/running OME suspensions for over 20 years now. I was as nervous as anyone when they switched from OME to Dakars, in fact I personally bought the remainder stock of all of the original springs! Fast forward 11 years now and we see less than or the same number of Dakar warranties as we did the original OME springs. The spring eye inserts did have some press-fitment issues right off the bat but the logic was sound and we've had zero calls about those in ~5 years now?
How much highway do Australians see? Well, that is a tough question. Given 85% of their population is in major urban areas, it's a ton of pavement to get to the "Outback". I've crossed Australia several times now by vehicle and we did that on approx 50% off-road, much of that off-road being brutal corrugated roads that shake fillings out!