Commonwealth
Go to Federal Register of Legislation and navigate to the Act you are researching, by selecting either 'Acts in Force' from the menu or searching for the Act:

Once you have found the Act you are researching click on the link 'View Series':

This will give you a list of the Act at different points in time. The archive has consolidations dating back to 1997. Using the information in the the “Start Date” and “End Date” columns, find the version of the legislation for the point-in-time relevant to your research. Select the link to the relevant version.

Once you have opened the Act, go the endnotes section and look at the amendment history to see the exact commencement dates for certain provisions.