keep guessing, or........
CASA Aircraft Register Procedures Manual
2.4 What is an Aircraft for the Purposes of Registration?
For the purpose of registration an aircraft is:
Any machine or craft that can derive support in the atmosphere from the reactions of the air, other than the reactions of the air against the earth’s surface, as defined in the Civil Aviation Act 1988; and that is intended to be used as an aircraft
Whilst ultralights do not use the ICAO issued Australian prefix “VH” and they are not registered in accordance with the provisions of the CASRs, CARs or this Manual, ultralight aircraft are considered to be Australian aircraft.
Civil Aviation Act 1988 Part 1 Interpretation:
aircraft
means any machine or craft that can derive support in the
atmosphere from the reactions of the air, other than the reactions of
the air against the earth’s surface.
So the answer would be "when it is registered".