I'm sorry for coming into this discussion so late ... but I was having a similar problem to yours, catnap, riding with sneakers on my hybrid bike. The sneakers were reasonably thick-soled and I thought they would be OK, but over time I started experiencing numbness similar to what you you described.

I decided I really needed to try a real biking shoe with a very stiff sole. But, I wasn't quite ready to try a clipless system. After checking out what was out there, it looked like a pair of the lighter-weight "mountain bike"-style shoes would do the trick, which can be used with or without cleats.

So, just purchased a pair of Shimano M037W's (women-sized, imagine that!), and since they are last season's model , they were on sale They have very stiff soles, but still quite comfy, and I've only been out on them once so far but I think they might be just what I was looking for. I can also walk around on them fairly easily, which is nice to be able to do, say, if I stop for a cup of coffee somewhere.

They are SPD-compatible, so if I decide to try a clipless system I can use these shoes, but in the meantime I'm still experimenting with toeclips and, more recently, powergrips. But I imagine I'll switch over to SPD pedals eventually if not sooner, now that I have the shoes!

Since you're in Australia I suppose you won't be in the midst of end-of-season sales like we are in the N hemisphere. You might have a hard time finding the M037's, they are being replaced with the M038's , which look a bit different but seem to be essentially the same shoe.

Best of luck, hope this helps!

take care, rosie