Ed: I've never cycled the route but have driven over most of it.
Dec-Jan is the main holiday season here. The highway is narrow and
winding and there will be plenty of traffic. Careful planning will find
you sections off the main drag.
Prevailing wind isn't likely to be a factor.
Don't miss Wilson's Promontory (important national park) in Victoria,
even if it's off your main route.
There's a difficult pass (narrow, steep, twisting) btwn Sydney and
Wollongong (next city south); plan to avoid this by a short train trip.
The pass is considered unsafe for bikes by the organisers of the annual
Sydney-W'gong ride, and the alternative road is closed. Once past
these few miles (on either side) the riding is fantastic.
Happy for you to contact me for any other suggestions
). Have a fantastic trip.