While it's a great idea, it won't work for a couple of reasons.
Firstly as already stated, most drivers seem incapable of driving safely within the current 100 / 110 kph speed limit. How on earth are they expected to cope with 140? Not until there is a radical improvement in testing and skill levels of drivers will it be possible to consider raising the speed limit to 140.
Secondly, many drivers seem to think it their God-given right to drive in the right hand lane on divided highways, like the Hume. Combined with the inability to drive at 100 / 110 kph, such behaviour will undoubtedly anger those drivers both capable of driving and willing to drive at the much higher speed. This is what contributes more to crashes than the actual speed involved. Not until "keep left unless overtaking" is rigorously enforced and drivers get over this "I can drive in whatever lane I like" mentality, it will be difficult to implement a 140 limit.
Speed doesn't kill, it is inappropriate speed that kills.