Where is the Data?
Most of the data is in the "MMSDM", on a public website called "Nemweb". The root URL is https://www.nemweb.com.au/Data_Archive/Wholesale_Electricity/MMSDM/. You can explore those folders to get a sense of the structure. The files you want are probably the ones like these:
(Note the month and years in the URL).
The filename mostly (but not always) corresponds to the table.
e.g. PUBLIC_ARCHIVE#DISPATCHPRICE#FILE01#202509010000.zip contains data for the DISPATCHPRICE table.
Danger
For some BID* and MNSP_BID* tables, the filenames are wrong.
e.g. PUBLIC_DVD_MNSP_PEROFFER_202407010000.zip is for table MNSP_BIDOFFERPERIOD not table MNSP_PEROFFER.
The large tables may be split into multiple files per month.
Older files may have a slightly different naming structure (e.g. _ instead of #).
Danger
AEMO do strange things with their URL encoding.
The _ and # in URLs may be double encoded.
The other folders contain scripts and the same data in other forms used to load the data into an Oracle SQL database. This is what AEMO expects market participants to do. However it is incredibly complex and error-prone. For example some scripts make assumptions about column order, but the data files do not necessarily have a consistent column order across months. The schema itself has changed over the years, and the historical scripts do not necessarily cope with that well. From personal experience, my advice is to not touch that stuff unless you already have extensive experience with PDR Loader and already have an Oracle SQL database.
The P5MIN_* tables are in both DATA/ and P5MIN_ALL_DATA/. As far as I can tell, the files are identical, other than the first row (which is metadata) and the filename.
Most PREDISPATCH* tables are extremely large. (PREDISPATCHPRICE is not particularly large though.)
The data for these tables in DATA/ is just a subset of the full table.
The full dataset for those tables is in PREDISP_ALL_DATA.
Later on there is an example of finding and using PREDISPATCH and P5 data.
The NEMDE folder contains some special tables, including price setter information.
The live (5 minutes old) data is available in /Reports/CURRENT. The file formats are more complicated because they are several CSV files with different columns concatenated into one file. You probably do not need that unless you are actively operating in the market.