From Wednesday, 25 August 2021:

Every Victorian aged 16 to 59 years will be eligible to access Pfizer vaccine at state-run vaccination centres. (Booking only, no walk ins). People who have already had one AstraZeneca will be offered the AstraZeneca vaccine as the second dose.

Children aged 12 years to 15: eligible for Pfizer vaccine if they are Aboriginal and Torres Strait Islander or have an underlying medical condition
People aged above 60 years will be eligible for Pfizer vaccine if they have specified underlying medical conditions (details can be found in source link below).