Jim:

In Australia you have 220-240 volt power similar to Britain. Your wiring system doesn't have a neutral, which is used in North America with its 120/240 volt system. The neutral enables the power system to have the equivalent of a transformer center tap, giving 120 volts. In dimmers that use it, it provides full voltage to run the dimmers circuitry. The big problem is many switches use a switching leg type curcuit that doesn't have a neutral available in the switch box. This I assume is a similar problem you have in the 240 volt circuits, there is no way to provide full voltage to run the dimmer, so they use what "leaks" through the light bulbs. This is not ideal because you can never turn off the circuit for safety reasons.