The lockdown will be extended until 12:01am on July 17 for anyone in Greater Sydney, the Blue Mountains, Central Coast, and the local government areas of Wollongong and Shellharbour.
The social distancing restrictions imposed on all other parts of the state will also continue.
Ms Berejiklian said the decision to extend the lockdown was a difficult one.
“We appreciate and understand the stress this means for individuals, for families, and, of course, for businesses,” she said.
“But what would be far worse is being in a situation where you have to live in and out of lockdown until that period of time when we have the vaccine available to us.
“That is not a way to live and we want to give our citizens the best chance of staying safe and healthy.”
Further restrictions were also flagged, limiting movement in key suburbs, such as outdoor exercising limits and travel zones.
“A next step could be limiting even further movement through the health orders,” Ms Berejiklian said.
“At the moment, obviously, we have broad rules as to why people are allowed to leave the house and allowed to exercise, but if we need to, Health may provide advice on further restriction of movement.”
The Premier said the next nine days were “perhaps the biggest challenge our state faces since the beginning of the pandemic”.
Students returning to school next week in Greater Sydney, the Blue Mountains, Central Coast, Wollongong and Shellharbour will face four days of home learning.
However, face-to-face classes will resume in all other areas of the state.
The schools have been closed to lessen the movement of parents and students in the community, but Ms Berejiklian said that campuses would be open for the children of essential workers.
“Of course those parents who are doing essential work, who have no option, children are welcome on the school grounds so as we did last time,” she said.
The Premier foreshadowed an increase in the number of infections tomorrow because of higher case numbers in the Fairfield, Canterbury-Bankstown and Liverpool areas.
While the outbreak started last month in Sydney’s eastern suburbs, the city’s south-west has emerged as a key area of concern for health authorities.
“Can I say to the communities in those area, many have a similar background to me, please don’t mingle with family. I haven’t seen my parents since the lockdown started — it is hard,” The Premier, who has Armenian heritage, said.
“Can I say to everybody don’t mingle with family, don’t think it is OK to visit your cousins or have sleep overs. Please, in those three local government areas limit your movement.”
NSW Health confirmed that only 13 of the new cases recorded in the 24 hours to 8:00pm yesterday were in isolation prior to their positive COVID-19 test result.
Additionally, 18 of the locally acquired cases are linked to a known case, 11 are close contacts, and seven are household contacts of previously confirmed cases.
Chief health officer Kerry Chant said the idea that the virus only affected the elderly was not true.
She said NSW had 37 COVID-19 patients in hospital, including 14 under the age of 55. Of those, eight are under the age of 35.
“It is a bit of a wake-up call to young people,” she said.
“The Delta strain is not a mild disease. It can be mild in some but for many it can lead to hospitalisation and death.”
There are currently two people in ICU on ventilators.
More than 45,000 people were tested overnight, an increase from 32,000 on Tuesday.
Several new infections were linked to an illegal party held at Meriton Suites in Waterloo, bringing the total number of cases in that cluster to 15.