Whilst I had my first vaccine less than a week ago there's no guarantee that they will definitely open up to 30-39 next. It could be 35-39. Also they could decide with the outbreak in Sydney that it's important to get AZ hesitant over 60s that haven't had any dose yet vaccinated ASAP (as they are far more vulnerable than most under 40s) and open up Pfizer to everyone 40+ next (with those who have had an AZ already without clotting required to get AZ for their second dose).
Once Pfizer is opened up to under 40s the bookings may go fast. Now that I have had my first AZ I don't have to worry about that. I already have a booking for my second AZ dose late September and the nurse said that it could easily be brought forward to 10 weeks after the first if there was a strong need to do so, say if I happened to get an exemption to travel overseas for instance. She told me that days before the PM started recommending the vulnerable in Sydney get their second AZ dose from 8 weeks.