Premier Gladys Berejiklian spent most of Tuesday locked in discussions with health experts to gather information on "what next week looks like", ahead of a crisis cabinet meeting.

It's believed the government decided to extend the two-week lockdown on Greater Sydney, Wollongong, Shellharbour, Blue Mountains and the Central Coast, due to end on Friday, by another week to 11.59 on July 16.

With the new school term due to begin next week, students in those five regions are likely to be mostly learning from home. The children of essential workers will be able to attend schools, as has happened in earlier lockdowns.

Students outside those regions should be able to go back to school.

NSW recorded 18 new COVID-19 cases in the 24 hours to 8pm on Monday, bringing the total number of people infected in the latest outbreak that began on June 16 to 330.

While it was a drop from 35 cases in the previous day, the number of people who were in the community while infectious is still alarming health experts.

That includes seven of the new cases announced on Tuesday involving the virulent Delta strain of COVID-19.

Chief Health Officer Dr Kerry Chant has previously flagged that the lockdown's success will be judged on this metric.

Ms Berejiklian has said the government wants this lockdown to be the last.

The goal is to ensure a high proportion of the population gets vaccinated in future weeks and months.

The lockdown decision will be announced at the premier's regular 11am update on Wednesday.

Meanwhile, modellers, economists and public health experts from the Australian National University and the University of Melbourne have crunched the numbers and found longer lockdowns benefit the economy in the longer term.

"The key point here is not to think about the economic costs over a period of a couple of weeks, large as they are, but rather to consider the costs over a period of months if community transmission continues," University of Melbourne Professor Tom Kompas said on Tuesday.

According to St George Bank economists, the lockdown has already cost the economy about $1.5 billion.

Meanwhile, NSW Health says COVID-19 had been contracted by Commonwealth Bank customers at the Roselands branch in Sydney's southwest, a site that had been previously flagged as a venue of concern.

Anyone who visited the branch at various times between June 28 and June 30 must get tested and isolate for 14 days, regardless of the result.

More than a dozen other health alerts were issued on Tuesday night for venues - mostly in Sydney's west as well as train routes.

Harris Farm at Bondi Beach, St George bank at Riverwood, McDonald's and Woolworths at Bonnyrigg and Big W at Menai are among the venues with details and advice available on the NSW Health website.

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