Thanks for the story. I am sorry that this has stricken you at such a young age; most of us are older, though we have a fair number of members in their 30s and 40s.
IVIg has never had much of an effect for me, and apparently it can stop working, as possibly evidenced in your case. Other individual cases report that switching the brand, upping the dose, or increasing the frequency can help. Unfortunately, there isn't much of a consensus on these things. My own doc says the brand makes no difference, while at least two of our members here have experience to the contrary.
I would encourage you to read this MMN overview, which reports that plasmapheresis isn't considered a good treatment any more. But like I said, there isn't much consensus; my first neuro wanted me to undergo plasmapheresis, but then I sent myself to the Mayo Clinic for more authoritative opinion--at least that's what I hope, since my first neuro had never seen a case of MMN, and the Mayo sees as many as anybody does. The Mayo doc recommends IVIg at increased doses, then rituximab if that is unsatisfactory.
I do not know what your options are in Australia; can you arrange to see someone with more experience with MMN? We have so many Aussie members, maybe I should start a topic for you guys (maybe one for each country!) in order to share localized advice, experiences, and tips on treatment options...