The move came a day after the European Medicines Agency said unusual blood clots should be listed as a very rare side-effect of the AstraZeneca vaccine.
Australia’s Chief Medical Officer, Paul Kelly, reiterated the risk of clots was extremely low.
He said: “It’s only been found in the first dose of the AstraZeneca vaccine, usually within 4 to 10 days after that vaccine.
“But it is serious, and it can cause up to a 25 per cent death rate when it does occur.”
The government said health providers should only give the AstraZeneca vaccine to adults younger than 50 when the benefit clearly outweighs the risks, he added.
Italy and Britain joined other countries in suggesting age limits for AstraZeneca’s vaccine.
It comes after European regulators reaffirmed that the benefits of the jab still greatly outweighed the risks.
Those who have already had a first AstraZeneca dose without any serious adverse events “can safely be given their second dose,” Dr Kelly said.
Prime Minister Scott Morrison said the updated advice will delay Australia’s inoculation timetable.
It means that all adults are unlikely to be offered a vaccine by October as originally hoped.
“There will be a re-calibration of how the programme will need to be adjusted,” Mr Morrison told reporters in Canberra.
Australia has ordered 20 million doses of the Pfizer vaccine and Mr Morrison said the government was in talks to increase the order.
Those over 50 who opt for the Pfizer vaccine will have to wait, the government said.
Australia had planned to use the AstraZeneca vaccine for the vast majority of its near 26 million population.
Its immunisation campaign was already behind schedule only delivering 670,000 jabs by the end of March after promising to rollout four million.
Australia had been looking to ramp up the immunisation effort, underpinned by plans to make 50 million doses of AstraZeneca vaccine in Australia, produced by CSL Ltd.
Australia began vaccinations much later than some other nations because of its few infections, which stand at just under 29,400, with 909 deaths, since the pandemic started.