GB,
To give a specific routing before the day of flight is not possible because of the unknowns - most importantly wind and weather. However, as previous poster has listed, Great Circle mapper is a great website and will give you a plot of the direct track/shortest distance between the two airports.
Just type in LHR-KUL-SYD into the box and you'll see.
For the KUL-SYD portion, there are daily published "Flex Tracks" by the Aussies that the airlines use - these again depend on wind and weather.
You'll get a good general indication though from that website.
Enjoy your trip.