Australians under lockdown have been urged to receive a second AstraZeneca dose within eight weeks of their first jab.
Sydney is in the grips of an ongoing outbreak, with NSW reporting 38 new local coronavirus cases as the city and surrounds prepare for at least a third week of stay-at-home orders.
A 12-week gap between AstraZeneca jabs has been recommended as the most effective way to protect people.
But Prime Minister Scott Morrison wants people in the worst-affected areas who have received their first jab to make second dose appointments closer to eight weeks.
He said his latest recommendation was consistent with the advice of the expert immunisation panel Australian Technical Advisory Group on Immunisation (ATAGI).
In the three Sydney councils with the most infections, about half of those aged over 70 have received their first dose.
An extra 300,000 doses of AstraZeneca, which is recommended for people over 60, and Pfizer vaccines will be made available for New South Wales.
Nancy Baxter, the head of the University of Melbourne's school of population and global health, said 12 weeks remained the recommended AstraZeneca interval.
"We don't really know how effective it is at eight weeks," she told the ABC.
Epidemiologist Mary-Louise McLaws said the first AstraZeneca dose offered 33 per cent, while a second jab increased the figure to 60 per cent.
Professor McLaws supports bringing second doses forward but said the need for a booster shot later was likely.
"If we start it moving away from that 12 weeks then your antibody response may not be as good as it could be," she told the ABC.
"But given that we are in dire straits at the moment with Delta, we don't want it to go across any state borders, we don't want to go to the elderly, it's a good idea."
State and territory leaders will meet with the prime minister during a virtual meeting on Friday with the rollout remaining high on the agenda.
National cabinet will also discuss Australian Health Protection Principal Committee (AHPPC) research on alternative quarantine arrangements for vaccinated Australians.
An agreement is expected on mandatory jabs for disability support workers after the AHPPC recommended following a similar order imposed on aged care workers.
The prime minister will also talk about vaccinating fly-in, fly-out workers with premiers and chief ministers.
