As a good overview of the medical arrangements I recommend the on-line version of the official history of the Australian Medical Services. The relevant chapter can be found here:
I shall endevour to post from this a list of the locations of the principal medical units of Fifth Army.
The general scheme was that casualties were taken down by stages to Main Dressing Stations, which were usually operated by the Corps. They were sent to various Casualty Clearing Stations depending on whether they were walking cases or stretcher cases. A special arrangement in place at the beginning of August was a group of CCSs at Brandhoek which was intended to provide surgical operations as close to the front as possible. Eventually this turned out to be too close and they were moved back.
As always, the best-laid schemes o' mice an' men Gang ait agley, and casualties often ended up at some other station.