Another option is to go to Vancouver, Canada, and do it there. It will nearly cost you the same as the plane is only $240 p/h by the time you add the airfare there. Basic endo is around $2000 aussie dollars. Weigh it up with accommodation and flights within Australia and i think its pretty comparible, but you get some international flight time, and some duty free booze. Here is a link for you to check out...
Floatplane Training! FloatSafety Seaplane Training, Floatplane Training on Canada's Beautiful West Coast
I am not sure what requirements there are with CASA upon arrival back to Oz if you go this option, but this is stated on the above website...
The process for a foreign licence holder to obtain a seaplane endorsement in Canada is relatively easy. You must first obtain a Limited Term Pilot Licence from Transport Canada. Fill out the Limited Term Pilot Licence section and present it, your pilot licence and valid medical to a Transport Canada Office during business hours. It will cost $40 CAD and is valid for 90 days. It grants you flying privileges here in Canada. When you complete your seaplane training, return to the Transport Canada office with your completed endorsement form and $30 CAD for the fee. They will endorse your Limited Term Licence with the seaplane rating.
When you bring that document home to your own aviation authority, it is up to them to decide whether they will accept it or not and transfer the endorsement to your licence. They may require you to do a check ride or have completed a minimum number of flying hours to honour your Canadian endorsement, or they may just sign it off no problem. Canada is an ICAO state so you would think it should be acceptable. We have tried contacting the Aviation Authorities of Britain and the USA and have yet to receive a qualified answer. The best thing to do would be to inquire at your local aviation authority office and find out before you arrive here.
I hope this gives you another option to think about...