I'm not sure if this is relevant to the IREX exam which I'm preparing to undertake next week, but while doing some practice exams I noticed the YSSY ILS for 34R has two entries:

S-I ILS (3.6% MAP) 200 (207-08)
S-I ILS (2.5% MAP) 460 (447) 2.2.

I assume that the first assumes a capacity of climbing at a minimum of 3.6% on the missed approach the latter the minimum of 2.5% for which all missed approaches have been surveyed. I haven't seen this in other ILS approach charts (although I haven't gone through everything in the three DAP volumes) and it doesn't have it for the other ILS approaches at YSSY.

Is this something to do with noise in the Sydney basin perhaps? And for 34L it requires a CAT I ILS SA or Cat II SA which I think VH-OZY doesn't have

It's not Sydney only, other examples are around e.g. Canberra. The reasoning for the two (or more) DA's is based on terrain clearance from DA to obstacles or terrain on the missed approach path. The lower the DA, the greater the climb gradient required to clear the obstacles or terrain. Missed approach climb gradient is usually taken to be 2.5% however, the designer of the approach can give you a lower DA providing you can make a higher climb gradient on the missed approach.