Is it correct to put two strokes on an australian dollar sign?

This is how I was taught it at school in the late 70s.

I have noticed over the subsequent several decades that the rule is not so strict as it was. I think nowadays Americans and Australians alike are more and more often seeing the $ symbol with either one or two strokes as it occurs in the mind of some faceless software designer somewhere, and that this has confused the issue. I think when I write it with pen and paper, that I actually vary.

I notice in Cunctators link that the educational material for the changeover indicates 4 documents with a double line symbol…

Why is it that the dollar sign does not appear on the currency? The name is spelled out, but the symbol is not shown (same for Canadian $).