To volunteer as a marshal for the Australian V8 Supercars events you should register with v8supercarsvolunteers.com.au This web site was specifically organized to be a one stop shop for marshals interested in volunteering for Aussie V8’s.
To my knowledge the web site may not cover international V8 Supercars events like the New Zealand races in which case the local club there organizes their own marshals. Other countries may also have a similar arrangement.
Another avenue to register may be through your local club within Australia of which Confederation of Australian Motor Sports (CAMS) is probably the largest, but by no means the exclusive one. There are a number of clubs in each state that may or may not be CAMS affiliated that provide marshals for events in their area.
If you have any relevant information you wish to share on this subject please do get in touch.