Hi Tommy,
It is not iris japonica (having just gone through the process of identifying that one myself, with the help of this forum -
The thread "Help identifying flowering plant" in
Plant ID forum). It strongly resembles a dietes iris (
The thread "dietes iris?" in
Irises forum), but doesn't look quite the same as the dietes grandiflora that I've seen. My understanding is the key difference between dietes iridioides and grandiflora is that grandiflora has larger blooms (about twice the size), is much taller (up to 1m) and the blooms last for about 3 days (as opposed to iridioides, which last a single day).
Is it possible that it is a hybrid within the dietes genus?