Whilst I have seen Louvre Vans such as the NLGX mentioned as the origin of the NDMX, some research and debate has led me back to the NOBX open wagon. See http://www.railpage.com.au/f-p1595453.htm#1595453 for details.

They were easily distinguished from other spoil wagons in that they were the only ones painted blue, however recently a few have been repainted bright orange.

Kitbashing the IDR Castings (ex Scaleways) NDCH is probably the easiest way to model these wagons.