Hi Overland Man

, I will try and add some sense to your dilemma.
The shortest distance between two points on earth is not necessarily as simple as it seems.
You would have thought that to go from Sydney (latitude 33 south) to Johannesburg (latitude 25 South), you would fly very close to a westerly heading.
This is called a rhumb line track and without going into too much detail, represents a constant track on a map.
The shortest distance between two points on earth is called the great circle track, and is not a constant bearing. On a standard projection map, great circle tracks will appear to bend towards the poles. When departing Sydney for Joburg, the initial compass track would be 230 degrees (SW). Just before arriving in Joburg, the compass track would be 315 degrees (NW).
The best way to picture this is to consider two cities, city A (latitude 80N, longitude 90W) and city B (latitude 80N, longitude 90E). Because they both have the same latitude and are on "opposite" sides of the eath, you would think you fly due east or west to get there. However, if you fly straight over the pole (ie due north, over the pole, then due south) the distance is much much shorter!
You are also correct that winds play a part. It depends on the time of year, but generally upper winds in Australia are predominantly westerley with a strong jetstream (up to 200 knots at 30,000') dominating southern Australia, more so during winter. This means westerly flights may remain slightly north or south of the great circle track to minimise headwind.
The best way to make sense of this would be to get a large earth globe and pull a string tight between sydney and joburg. It will all make sense then.
I hope this hasn't confused you more
