Some stores there must sell AC voltage adapters for travelling to countries that run 120VAC vs. your 240VAC; I know the such adapters are readily available here for those travelling out of the country to face similar voltage differences. Perhaps the physical AC plug configurations on the adapters there are reversed, and physically wouldn't solve your issue - perhaps buy from a North American web site... They're nothing more than a small step up (or down as required) transformer with the corresponding AC plug configurations.
Once you have an adapter, the time to charge wouldn't change as compared to using 120VAC. Using the AC charger, Garrett states 8 hrs for a full charge, and using the cigarette lighter to charge, they claim 4 hours if the car is running (13.8VDC alternator charging voltage) or 8 hrs (12VDC just the car battery) if the car is not running.
That being said... It strikes me that both the AC and the cigarette lighter charges are not actually designed for battery charging; They just supply an unregulated DC voltage to the batteries, forcing the batteries to take the charge regardless of their charge state, heat, over-charging etc. If my assumption on the chargers' design is correct, this will degrade the charge/discharge life-cycle of the batteries.
I just received my new Infinium c/w rechargeable batteries, and I am going to buy a battery charger designed to recharge NiMh AA cells; The type that your commonly see for sale c/w rechargeable batteries for your electronic devices, digital camera etc. - they can be found in styles that will charge up to 8 batteries at a time. This will mean I have to remove the batteries from their holder however, vs. plugging the Garrett chargers into the batter holder.
If you can find a similar NiMh AA battery charger there, it would remove your need to find a voltage adapter as it will be designed for your 240VAC, and I suggest that you only rely on the cigarette charger if you run dry unexpectedly. The alternative to the cigarette lighter is to carry a spare set of alkaline batteries for such occasions, and you'll never have down time waiting for a charge if your rechargeable batteries should run dry.
I hope that helps...