Since it doesn't get that cold in Melbourne, eg 2 degrees C would be as low as it gets, if there is a wind, ambient temp is almost always higher, but then wind chill gets you.
as of a week ago, i only had summer weight gear. Lightweight spray jacket in yellow (squishes to the size of a tennis ball) knicks, and short sleeve tops.
However I decided to buy some 'winter' gear before the big rushes. My legs I figure for now will last through spring without any set of 'longs'. However I want to avoid getting a chest cold as well. So in these cooler mornings as summer is dissappearing, i got some Netti "sleeves" which work well with the summer weight tops. Eliminates the chill from the exposed arms. I figure these would be fine down to 9degrees by themselves (I don't feel the cold that much).
For anything much colder, or if there is a breeze out, I bought a LineBreak Long Sleeved T-Shirt to wear under a summer jersey. These are a tight fitting "compression" top. The arms are a slightly heavier spec lycra or whatever, than the chest so I figure it would be good for poor weather cycling - and still breathes well.
If it is raining, i'm going to stick with my lightweight spray jacket, as all I care about keeping dry and warm is my chest and back. If this jacket doesn't turn out to work that great in winter I will evaluate something else...