ATAGI have stated that AZ can be used as a booster:
The Australian Technical Advisory Group on Immunisation (ATAGI) have developed recommendations on the use of additional (booster) doses of COVID-19 vaccine.
www.health.gov.au
"Comirnaty (Pfizer) is recommended as a single booster dose, irrespective of the primary COVID-19
vaccine used. Although not preferred, Vaxzevria (AstraZeneca) can also be used as a booster dose
in the following situations:
• For individuals who have received Vaxzevria (AstraZeneca) for their first two doses if
there are no contraindications or precautions for use.
• If a significant adverse reaction has occurred after a previous mRNA vaccine dose
which contraindicates further doses of mRNA vaccine (e.g., anaphylaxis, myocarditis).
ATAGI will make a recommendation on the potential use of Spikevax as a booster in due course"