Australia will have to vaccinate 80% of its adults against Covid-19 before it can consider reopening its border, according to Prime Minister Scott Morrison.

Mr Morrison announced a four-stage plan to greater freedom in Australia, which is currently in phase A, also known as the suppression phase, with large parts of the country plunging in and out of lockdowns to stamp out the coronavirus.

Australia had handled the coronavirus crisis much better than many other developed countries, with just over 34,000 cases and fewer than 1,000 deaths, but that has been achieved largely by sealing its border to all but a trickle of people since the pandemic began.

Mr Morrison said the border would be gradually reopened in phase C of the plan, when 80% of adults have been vaccinated. About 18% of adults have been vaccinated already, under a campaign that got off to a slow start.

"We will lift all restrictions on out-bound travel for vaccinated Australians," Mr Morrison told a news conference, referring to phase C.

"There will be a gradual reopening of inward and out-bound international travel with safe countries," he said.

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Mr Morrison did not give a timetable for the plan but expressed confidence that phase B, or 70% of the population vaccinated, could be reached by the end of the year.

"Lockdowns in phase B are less likely, but they are possible," Mr Morrison said.

Australia has opened a travel bubble with New Zealand, which has also contained the virus successfully by sealing its border and with effective testing and tracing of the few cases that have cropped up.

Australia is in talks with Singapore on a similar travel bubble plan.

Army to enforce Sydney lockdown

Police in Sydney have requested military help to enforce lockdown, as infections reached a new record.

Commissioner Mick Fuller said New South Wales police had asked for 300 Australian Defence Force personnel to be deployed "to boost its operational footprint".

The city of five million people is in its fifth week of a lockdown that is set to run until the end of August.

Stay-at-home orders have failed to reduce new infections to zero, and compliance has been patchy.

Anti-lockdown protest in Sydney

Sydney residents are only allowed to leave their homes for exercise, essential work, medical reasons, and to shop for necessities such as food.

But for weeks, parks and beach promenades have been filled with Sydneysiders drinking coffee and chatting with friends.

Police have increasingly been doling out fines to those violating the restrictions and Mr Fuller said those efforts would be stepped up in the coming days.

Last weekend thousands of people gathered in central Sydney to protest against the measures, and further demonstrations have been mooted.

Police have also requested more powers to shut down businesses that they say are not abiding by rules on social distancing.