The decision affects 250,000 doses of the vaccine produced at an AstraZeneca facility in Italy.
Italy is the first EU country to use the bloc's new regulations allowing exports to be stopped if the company providing the vaccines has failed to meet its obligations to the EU.
The move has been backed by the European Commission, reports say.
AstraZeneca is on track to provide only 40% of the agreed supply to member states in the first three months of the year. It has cited production problems for the shortfall.
In January, then Italian Prime Minister Giuseppe Conte described delays in vaccine supplies by both AstraZeneca and Pfizer as "unacceptable" and accused the companies of violating their contracts.
The EU has been widely criticised for the slow pace of its vaccination programme.
Under the EU vaccine scheme, which was established in June last year, the bloc has negotiated the purchase of vaccines on behalf of member states.
There has been no official comment on the Italian move by Australia, the EU or AstraZeneca.
Australia began its vaccination programme last week using the Pfizer/BioNTech vaccine. It was due to start inoculations with the AstraZeneca jab on Friday.