The adjusted advice comes after the Australian Technical Advisory Group on Immunisation (ATAGI) held its weekly meeting on Monday evening amid the worsening COVID-19 outbreak in Sydney.
Across the country, an interval of between 8 and 12 weeks is recommended between first and second doses of the AstraZeneca vaccine to give long-lasting protection.
But in areas like Sydney, which recorded 81 new coronavirus cases on Tuesday and one additional fatality, the group has now advised "shortening the gap between first and second doses will bring forward short term protection, which is expected to be beneficial in outbreak situations".
ATAGI noted a single dose of the AstraZeneca vaccine reduces the risk of infection against the Delta variant by just 30 percent.
In comparison, two doses give significantly stronger protection, reducing the risk of infection by 67 percent and the risk of hospitalisation by 92 percent.
Three out of four new COVID-19 cases in NSW are household contacts
"On this basis, an interval of between 4 and 8 weeks between the first and second doses of COVID-19 Vaccine AstraZeneca is preferred in an outbreak situation," the group wrote in a statement.
Across the country, the AstraZeneca vaccine is recommended for use for people over the age of 60, due to an extremely rare chance of severe blood clots in younger groups of the population.
But ATAGI has now said residents impacted by "a significant COVID-19 outbreak involving the Delta variant" should "re-assess" the benefits of being vaccinated with the AstraZeneca vaccine, versus "the rare risk of a serious side effect".
"In outbreak settings, the benefits of COVID-19 Vaccine AstraZeneca are increased compared with non-outbreak settings," the group wrote.
"When the virus is spreading in the community it is critical that as many people as possible are vaccinated as quickly as possible."
"Given the changes to the risk-benefit equation, ATAGI recommends adults under 60 years who do not have immediate access to Comirnaty (Pfizer) should re-assess the need for vaccination with AstraZeneca given these greater benefits."
The group said the risk of contracting COVID-19 had increased in Greater Sydney due to the expanding outbreak.
The "cumulative risk" to Sydney residents is approximately 10 cases per 100,000 people and is increasing by 2 cases per 100,000 each day.
The risk rises to more than 100 cases per 100,000 people in the Fairfield Local Government Area where the majority of cases have been recorded in recent days.
Prime Minister Scott Morrison said the change in advice reflected the changing risk of contracting COVID-19.
"It’s based on an assessment and evaluation of risk. What we’ve asked the Doherty Institute to indicate to us is what that balance of risk is and how you can then adjust measures to suit that risk to ensure that we meet all the objectives we need to have as a country.
"What is important is that we continue to move as quickly as we possibly can (to get the population vaccinated)."
He said in a week's time, Australia is on track to surpass the mark of 10 million vaccine doses administered.