"Pleasingly 17 of these cases have already been linked," NSW Premier Gladys Berejiklian said.

"Two are still under investigation."

NSW chief medical officer Dr Kerry Chant said one of the cases under investigation lived in Sydney's eastern suburbs while the other worked in that area.

"In terms of the cases, seven of the cases were in isolation for their infectious period," Dr Chant said.

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"At the moment we are still seeing some cases that were potentially still in the community while infectious, but what we are hoping is that because of the lockdown the chances for that transmission will have been reduced considerably."

The cases include a Rose Bay Secondary College student, some cases linked to a Marrickville seafood wholesaler and others through a birthday party held in Hoxton Park.

Dr Chant said that anyone whose symptoms changed or evolved should consider re-testing.

"One negative test does not predict what may eventuate later that evening or in the next few days," she said.

Ms Berejiklian said she expected more cases to arise, "especially from household contacts". She urged anyone whose second AstraZeneca dose was pending to ensure they received the second jab.

"I want to assure people that the NSW government is already looking at what life will look like after the lockdown," she said.

Treasurer Dominic Perrottet confirmed at the Tuesday press conference that Dine and Discover vouchers would extend to the end of August as a result of Sydney's lockdown.

Lockdown continues for greater Sydney, including the Central Coast, while masks are mandatory indoors across regional NSW.

The figures came after NSW Health named a number of venues of concern in Sydney with exposure windows extending back as far as June 21.

More than 67,000 people were tested in the 24 hours to 8pm on Monday as demand for screening surges.

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