SYDNEY (REUTERS) - Australian authorities on Friday (July 9) pleaded with Sydney residents to stay at home, warning a three week lockdown may be extended as they struggle to control a Covid-19 outbreak, with the city reporting its the biggest rise in local cases for the year.

Hundreds of extra police patrolled parts of Sydney to enforce the city's lockdown orders imposed to stamp out an outbreak of the highly infectious Delta variant which now has a total of more than 400 cases.

"New South Wales (state) is facing the biggest challenge we have faced since the pandemic started," state Premier Gladys Berejiklian told reporters in Sydney. "At the moment the numbers are not heading in the right direction."

"Please do not leave your house. Do not leave your home, unless you absolutely have to," Ms Berejiklian said.

Forty-four locally acquired cases were reported on Friday in NSW, Australia's most populous state, eclipsing 38 a day earlier, with 29 of those having spent time in the community while infected.

There are currently 43 cases in hospital, with 10 people in intensive care, four of whom require ventilation.

The rise in cases is despite a two week lockdown which has now been extended to a third week ending July 16.

In Sydney's southwestern suburbs, now the epicentre of the outbreak, streets were virtually deserted on Friday, with groups of police patrolling the suburbs.

In efforts to further restrict people's interaction, from Friday evening public gatherings will be limited to two people and residents will only be allowed to travel 10 kms (6 miles) from their home.

Ms Berejiklian ruled out any suggestions of "living with the virus" considering the low vaccine coverage in Australia.

"If we choose to live with this while the rates of vaccinations are at 9 per cent, we will see thousands and thousands of hospitalisations and deaths," Ms Berejiklian said.

Although Australia has fared much better than many other developed countries in keeping its Covid-19 numbers relatively low, its vaccination rollout has been among the slowest due to supply constraints and changing medical advice for its mainstay AstraZeneca shots.

Australia has relied on the AstraZeneca vaccine to fight the pandemic, but now limits it to people aged over 60 in its mass vaccination centres due to the risk of bloodclots, leaving short supplies of Pfizer for people 40 to 60 years of age.

Roughly a quarter of hospitalisations in the Sydney outbreak are people aged 35 and under, an age bracket not yet eligible for vaccinations under the government's programme.

Australian Prime Minister Scott Morrison on Friday (July 9) said Pfizer will increase Covid-19 vaccine delivery to about one million doses a week from July 19, more than tripling shipments, as Sydney battles its worst outbreak of this year.

As many as 4.5 million Pfizer Inc doses that were expected to arrive in September will become available next month, Mr Morrison said.

"We've had challenges over the course of the last four months but we're hitting those (vaccination rate) marks now," Mr Morrison told broadcaster Nine News.

Pfizer said the total number of 40 million doses it is contracted to deliver to Australia over 2021 had not changed.

Mr Morrison said Pfizer was bringing forward vaccine deliveries to Australia from September to August.

The state of South Australia would meanwhile run a trial of home quarantine, Morrison said, following criticism of the two-week hotel quarantine required of all people entering the country.

Hotel quarantine virus leaks have been the source of Australia's community outbreaks, after the country effectively eliminated the virus by shutting its international borders.

Dr Jamal Rifi, a general practitioner from Western Sydney, the epicentre of the current outbreak, said many residents had misunderstood the Delta variant risk but were gaining awareness as infection rates rose.

"Many members of the local community have been identified as sufferers of the virus and many loved ones are in hospital or even ICU," said Dr Rifi.

"The word has spread in the community and I reckon you'll see more people now limiting their movement, taking this Delta variant more seriously."