Ian,
I don't think anyone has ever actually set up a local SA Standard for layouts anyway as everyone has their own way of doing things. Locally most of the electrical stores work on mm
2 for measurements rather than AWG. See
http://www.engineeringtoolbox.com/awg-wire-gauge-d_731.html for a comparison of AWG to mm
2I'm also just going to give what I use rather than a standard.
For the main DCC Bus I use 2.5mm
2 House Wire (also know as General Purpose wire). Normally I use red and black insulated wire
For feeders I've used both 0.2mm
2 and 0.5mm
2 on different layouts. The smaller size is easier to solder to the track.
For Peco point motors 0.5mm
2 is good but I've used 0.2mm
2 as well.
Auxilly DC power bus I normally use 0.5mm
2 with 0.2mm or less feeding off that as needs be.
Control panels I've generally used 0.2mm.
I try to use a color code and stick to it.
As to suppliers, I normally use East Rand Electrical in Benoni, but any almost any of the electrical wholesalers should have these sizes available. If you have two or three in a close vicinity shop around a little as prices could vary.
Hope this helps,
Niel