In this case "unregistered refers to a nonVH plane eg RAA. Most VH aircraft will already have an ICAO number on their rego papers and this must be embedded into their transponder &/or their EC device.
From CASA Publication ADVISORY CIRCULAR AC 91-23v1.0
"8.2.5 24-bit address for EC devices used in unregistered aircraft
8.2.5.1 If you are using an EC device on an unregistered aircraft, you need to contact the Civil Aircraft Register <https://www.casa.gov.au/aircraft/civil-aircraft-register/contact-civil-
aircraft-register> and provide the following:
− Your contact details
− If applicable:
o Your aircraft's registration number
o Aircraft Manufacturer
o Aircraft Model
o Aircraft Serial Number
− The details for your EC device, including:
o Make
o Model
o Serial number.
8.2.5.2 CASA will allocate the EC device a unique ICAO 24-bit binary address to enable it to be used on multiple unregistered aircraft without re-programming. As mentioned in paragraph 8.3.2, CASA provides 24-bit binary addresses in several formats. If necessary consult the equipment manufacturer's documentation for the current format
to use for your equipment when converting the 24-bit address to the correct format.
8.2.6 Selling portable ADS-B devices
8.2.6.1 If you sell or dispose of a portable ADS-B device such as an EC device, you should ensure that the 24-bit address is cleared before it leaves your possession."