@david
As far as I can tell no adapter exists to convert Australian and New Zeland SodaStream Co2 threads to standard SodaStream threads, and this is not specific to just iGulu. A lot of info online about people complaining that AUS decided to use their own unique thread just to screw with everyone.
Loooked all over the place and I cannot even find a name or size for the unique australian thread.
So you most likely will not find an AUS SodaStream Thread to TR21-4 adapter. The only adapters I can find are AUS SodaStream to Standard Co2 like here. Which are designed to let you hook up a standard Co2 tank to an Australian SodaStream machine which will not help you.
Perhaps someone else has the name of the thread, but I had absolutely no luck finding it anywhere. The actual SodaStream market is fairly large, and even they have not created the adapter you are looking for as far as I can tell. I do not believe iGulu has a special model for Australia and New Zeland that allows the use of their unique SodaStream cannisters, so your best option would most likely be to just buy a standard Co2 tank, fill it, and get a standard Co2 tank to TR21-4 adapter and use that. In the long run it will be much cheaper than actual SodaStream cannisters.
Grab something like this here and use it with something like this here to hook it up to your iGulu. Those are the exact products I am using for my Co2 setup, and it looks like they ship to Australia. I am not really sure how easy it is to fill up a standard Co2 tank where you are, but a SodaStream cannister has 425 grams of Co2 and a 5Lb tank holds nearly 6 times as much and can be filled for half the price of a single cannister where I am at.
Sorry this does not directly answer your question, but it does give the only solution I was able to find to your problem.
iGulu team probably needs to make units that are unique to australia and new zeland, or document the difference directly on their page letting those customers know that their SodaStream cannisters will not work.