Hi Chris
Rookwood is a HUGE Cemetery, it is divided into sections each area administered by their own trust, so you have the Roman Catholic area, the Church of England area, Jewish , Independant (Methodist, Baptist Presbyterian) etc etc area. So Death Certificates may record a person being buried in the Church of England or maybe the Roman Catholic Cemetery at Rookwood.
As each Section have their own Office and keep their own records, if you only know that the person was buried at "Rookwood" but are not sure of which section, you have to contact each Office to find the location of a grave.
The Crematorium also keep their own records.
If you wish you could down load Google Earth (it is free) and it will give you an idea of the area Rookwood covers, it is the size of a normal suburb, full of roundabouts and one way streets.
Hope this helps
Cass