Hi Kassey and welcome to the forum. Lot's of Aussies here.
It's not uncommon for older back bulbs to be a little shriveled. Occasionally one of them will have fulfilled its usefulness and it will dry up and turn brown. As long as your general culture is good and the rootsare healthy etc. There's no cause for concern.
On a different note. Your Wilsonara is LOVELY! Very nice shape and colour.