NSW's chief medical officer is concerned about a missing link after cases of Covid-19 were found that have no links to current outbreaks.

NSW's chief health officer has admitted authorities "missed a chain" in their coronavirus tracing, as the state records 10 new cases.

Of those cases, four are in hotel quarantine and six were locally acquired with three linked to the Crossroads Hotel cluster in Casula, southwest Sydney.

The remaining three are still under investigation.

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"Our aim will be to find any crossover points with the list of venues in southwestern Sydney that have been identified," chief health officer Dr Kerry Chant told reporters on Thursday.

"Obviously we are concerned when we find cases that can't be linked back because it does indicate we have missed a chain."

Chant said the new cases associated with the Crossroads included a child who was a close contact of a previously known case.

The others linked to the Crossroads Hotel are a man in his 80s and a man in his 30s.

Since 8pm Wednesday NSW identified two new cases of the virus, including a teenager who attended the Hurricane grill at Brighton-Le-Sands on July 11, Chant confirmed.

The other was a man who flew from Melbourne to Ballina on flight JQ466 on Sunday July 12.

"The person was wearing a surgical mask," Chant said.

"The individual was screened on arriving in Ballina airport and as I said, since arriving, they (have been) in the 14-day self-isolation. Any potential close contacts are being followed up."

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Chant said the three locally acquired cases, which didn't appear to be linked to the Crossroads Hotel, were also based in southwestern Sydney.

Contact tracing experts are now working to determine how they became infected.

"We're not yet sure where they are linked to but we will re-interview people multiple times," Chant said.

Victoria recorded more than 300 cases of Covid-19 today, the state's highest total.

Chant said she was concerned about what's happening in the southern state but said it was up to NSW to "mop up any seeding".

"We can't become complacent about it, but we need to be optimistic and work together. We are as good as each individual part," she said.

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The news comes as The Australian reported the state government was moving toward tightening Covid-19 restrictions further to restrict further potential outbreaks.

It is understood senior government ministers met on Wednesday evening to discuss making changes to existing public health orders, which are likely to target restaurants and cafes.

This in on top of new restrictions which include dropping individual booking numbers from 20 to 10.

There will also be a cap of 300 for venues and every patron will be made to register their details with the establishment. Previously, this was only mandatory for the person doing the booking.

Dedicated hygiene marshalls will also be employed at venues across the state to ensure they are following government policy and people are social distancing.

Chant said NSW was at a "critical phase" and the community and businesses needed to work together.

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"People think their social groups won't infect them but the data tells differently," she said.