Difference between 10 NM MSA and Sector MSA
By way of clarification of the reason why a difference can exist, there is a requirement to place a buffer around the lateral boundary of each MSA sector. This buffer extends right around the reference navaid.
I'm not sure how closely Oz follows Pans Ops in this respect but PNG uses a buffer that is 5 NM wide and Australia won't use a wider buffer than that.
This buffer is also applied around the distance limit, whether that be 25 NM or 10 NM. Therefore, the area which is surveyed for the 10 NM MSA is a radius of 15 NM. This is obviously far larger than the buffer around the navaid, as used for the 25 NM MSA sector, because the only protection around the reference navaid is a radius of 5 NM.
Accordingly, an obstacle can exist at any distance greater than 5 NM, up to 15 NM and not affect the protection for the 25 NM sector MSA, but will certainly affect the 10 NM MSA.