The state opened up Pfizer vaccinations to everyone aged 16 to 39 on Wednesday, the demand causing the booking website to crash.

"The priority for us is to make sure that all final-year students have received their vaccination before they start sitting their exams on October 4," Education Minister James Merlino told reporters.

"So there'll be very strong communication to all students and staff across all our schools in Victoria, but a particular message to our Year 12 students, but also VCE staff and staff who will be supervising the VCE exams, we also want them to get vaccinated. So, please get vaccinated as soon as you're able."

Mr Merlino also said the state remained hopeful children aged 12 and up could get their first dose of a vaccine by the end of the year, dependent on the health advice.

Victoria has recorded 45 new locally acquired COVID-19 cases, and the state government's vaccination website has crashed as thousands of young people try to secure a booking.

Of the cases, 36 are linked to known outbreaks, while the source of the remaining nine infections are under investigation.

Twenty-eight cases were not isolating and infectious in the community.

The total number of active cases in the state is 538.

More than 830,000 vaccination bookings over the next four weeks opened from 7am on Wednesday, including 450,000 first-dose Pfizer appointments.

Within minutes of bookings opening, the website appeared to buckle due to high traffic.

Many young people reported receiving a "500 internal server error" after waiting for up to an hour in the queue to schedule an appointment.

Others made it through the queue, only to be told that they were ineligible for the Pfizer vaccine.

Those who phoned the state's coronavirus hotline were also unable to schedule an appointment.

"The Andrews Labor government has had months to get prepared and yet the confusion continues and the system crashes again," opposition health spokeswoman Georgie Crozier said in a statement to AAP.

"Getting vaccinated is our way out of lockdown, Victorians should have the confidence that they can book their appointment, get the jab and help our state back open."

Those aged 18 to 39 who have already made a first dose AstraZeneca appointment will be given the option to instead receive the Pfizer jab, while 16- to 17-year-olds will strictly receive the latter.

Those aged 60 and over will continue to receive AstraZeneca.

The expansion of the state's vaccine rollout is possible because of an additional 175,000 Pfizer doses from the Commonwealth government, imported from Poland.

Australian Associated Press