I remember reading about those floor boards and I think they don't recommend laying the 90mm boards over an underlay (I believe it's ok with the wider 150mm boards).
Personally I wouldn't lay timber floor boards directly onto a slab. I've been told by a few people who work on high quality homes that it's not usually done (I guess because the slab is never flat, and also due to the moisture in the slab). Not sure what they do in display homes. Most of the ones I've seen have tiles or a floating floor.
I also found this on the net. Interesting reading:
Feature T&G Timber Floor Installation
http://www.timber.org.au/Resources/DS3% ... 202005.pdfI would be interested to know if any others around here have laid timber boards directly onto a slab, and if there have been any issues.