House of Representatives.
Online availability of memoranda for House of Representatives Bills begins in 1901 through Parlinfo. The Federal Register of Legislation (from 1996) and AustLII (from 1980) have selected collections. In each case, locate the Bill first: the memorandum is associated with the Bill.
Many taxation-related memoranda from 1936 onwards have been made available online by the Australian Taxation Office through their Legal Database (see the GovPubs database for further details).
Senate
Online availability of over 1,500 Explanatory Memoranda (mainly for Bills introduced in the Senate and Supplementary Explanatory Memoranda for Bills introduced and amended in the House of Representatives) are available in the Senate Tabled Papers database (as the database can be difficult to use, the searching tips should be read first). Copies are gradually being added to Parlinfo.