You'll need a converter first and possibly an adaptor as well.
The converter is the most important because it will change the voltage from the Australian system to a voltage that your HydroFloss can handle.
Your HydroFloss will burn up quickly (almost instantly) if you plug it into a higher voltage system like they have in Australia and Europe and many other parts of the world as well.
An adaptor would be something that goes between the outlet and your converter to make sure the plugs match.
So you will need a CONVERTER first and possibly an adaptor as well.
This is the converter I use when travelling and it has worked very well for me.
An adaptor will be something you will have to search for, but it is likely that a local hardware or electronics store at your travel destination will have one if you can't find the right one here. The travel destination stores will have those that are geared for their local plug configurations.