Premier Gladys Berejiklian says she's pleased testing rates remain high and none of the new cases are linked to the quarantine hotel security guard who recently tested positive, sparking fears of a breach.

"Which is good news, but, of course, we're on high alert to make sure that there aren't any additional transmissions that we're not picking up," she said on Thursday.

"We're concerned about the level of community transmission which may be actually happening in southwestern and western Sydney, and we really need people in those areas in particular to come forward and get tested."

NSW Health said the five cases were diagnosed from 28,767 tests reported in the 24-hour reporting period until 8pm on Wednesday, compared with seven cases from 19,414 tests in the previous 24 hours.

Chief medical officer Kerry Chant said three of those cases were from southwest Sydney and locally acquired.

The two others are returned overseas travellers in hotel quarantine.

Of the three locally acquired cases, one is a household contact of a previously reported case associated with the July funerals cluster, bringing that to 74 cases.

One case is an outpatient at Liverpool Hospital, while links to other cases and the source of infection are being investigated.

There are currently 111 COVID-19 cases being treated by NSW Health with eight patients in intensive care and five being ventilated.

Dr Chant is also concerned about transmissions in southwest and western Sydney "where we continue to have one or two cases everyday pop up where we don't have an obvious source".

She stressed it was critical that high testing rates continued and the community abided by COVID-safe behaviours.

Authorities are still investigating how a security guard at the Marriott Hotel in Circular Quay contracted the virus.

"Can I stress that without divulging too much information that there's no evidence to suggest this person did the wrong thing at all but we have no indication there has been a quarantine breach," Ms Berejiklian said.

She defended the NSW quarantine system, saying it had processed more than 47,000 people and was allowing Australians to return home from overseas on behalf of the nation.

"With Melbourne out of the picture, Sydney is pretty much picking up the load and if we don't do it, number one, you won't have Australians being able to come back home and, secondly, those international flights will get cancelled and so I know we bear that responsibility and we do it we do it well," she said.

She also rejected calls from Deputy Premier John Barilaro to re-open the border with Victoria, saying she understands the "angst and frustration" but that the health of the state as a whole must come first.

"The last thing that I would want to see is the disease seeding in rural and regional NSW," she said.

Late on Thursday, Sydney bus drivers called off a planned strike after the NSW government agreed to their demands for a review of mandatory mask-use on public transport.

The Rail, Tram and Bus Union met with government officials at the Industrial Relations Commission in hopes an agreement could be reached to avoid a strike next week.

"The issue of mask wearing on public transport has been significantly elevated this week and the NSW Government has undertaken to review health advice and guidelines if community infection levels increase," union secretary David Babineau said.

Australian Associated Press