The ACT Chief Minister Andrew Barr was blindsided by the news Pfizer eligibility would be expanded to people aged 16 and above at the end of the month.
He was only made aware of the decision as the Prime Minister announced it on Thursday.
"No briefings have been provided to the ACT Government," he said.
"It is unclear if the Commonwealth will be providing additional supply of Pfizer to enable the appointments to be made."
Prime Minister Scott Morrison on Thursday said the vaccine would be open to the 16 to 39 age range from August 30.
"We agreed today, and the cabinet met today, to affirm that we will be moving to opening up 16 to 39 year olds for the balance of the program," Mr Morrison said.
But people in that cohort shouldn't try to book an appointment yet.
People aged 16-29 years will soon be able to register for Pfizer COVID-19 vaccinations. We know you’re keen, but don’t call our booking line or GPs – you will not be able to register or make an appointment yet. Information on how to register for the vaccine will be provided soon. pic.twitter.com/7qIIdPH79K— ACT Health (@ACTHealth) August 19, 2021
In the ACT, people aged 30 to 59 are eligible for Pfizer at government-run clinics and some GPs.
"The government is aware of the demand for the vaccine in this age cohort," Mr Barr said.
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"The current outbreak we are experiencing demonstrates that younger people are clearly more vulnerable to the virus given they have had limited access to vaccination at this point."
Since the onset of the outbreak in Canberra, young people have come out in droves to get vaccinated with AstraZeneca, widely available at pharmacies and general practice.
The ACT Pfizer hubs at Garran and the Airport are booked out until October 20 and 24 respectively.
Mr Barr has flagged that "workforce pressures" brought on by Delta strain outbreak could impact the vaccination rollout, if cases continued to spread and demand for testing remained.
The ACT government is working on a fourth mass vaccination hub to provide the Pfizer vaccine, with the Australian Institute of Sport among the locations under consideration.
"We hope to provide more information in the coming days," The Chief Minister said.
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