Hi Peter
A lot of confusion about this. AIP sets out the requirements in Gen 3.4 para 4.16.1 "within Australian airspace, "group form" is the preferred means of transmitting call-sign/flight number......" 4.16.2 "Group form is the grouping of numbers into pairs, or where a number ending in '00' is spoken in hundreds ....." 4.21.1 d. for recreation-category aircraft, the aircraft type followed by the last 4 characters of the aircraft's registration number ( eg "JABIRU THIRTEEN FORTY SIX"). note - the capitals are as per the AIP, not me shouting.
If the call sign was Jabiru 1200 the call would be "JABIRU TWELVE HUNDRED"
Hope this clarifies the situation
Cheers, Bill