Genomic testing has identified that a Sydney security guard who's tested positive to COVID-19 has the same highly contagious UK strain of the virus as a returned traveller who stayed at a hotel where he worked.

While no further cases of community transmission were recorded in NSW in the 24-hours until 8pm on Sunday, the quarantine period has been extended for some of those staying at the Sydney Sofitel.

The 47-year-old guard, who was diagnosed on Sunday, worked on March 6 at the hotel where the infected traveller had stayed on the 11th floor.

New South Wales Chief Medical Officer Kerry Chant said authorities had reviewed CCTV footage from the night the guard worked.

There was "not a clear and obvious breach" that explained what had caused the infection. The guard had no symptoms and had followed the required protocols, Dr Chant said.

"We haven't been able to find an exact source, an explanation for how this person became infected," she said on Monday.

"So we are testing everyone who worked, co-workers who worked with this gentleman to just check there is not someone else who may have been an intermediary that transmitted to this gentleman."

The infectious traveller did not open his hotel room door or leave his room while the guard was on shift, Dr Chant said.

NSW Health is re-testing everyone staying on the same floor as the returned traveller and extending the quarantine period for those on the 11th floor.

The 167 staff who worked at the Mantra Hotel in Sydney where the man worked an overnight shift on March 12 have also been contacted

Premier Gladys Berejikilian said she was relieved there had been no further community transmissions recorded.

She urged state governments across Australia not to overreact to the latest case and to "give us a chance" before making decisions on borders.

"There is no evidence of widespread transmission," she said.

It "dashes confidence everywhere if people can't rely on borders staying open", she said.

Meanwhile NSW has reached its target on the vaccine rollout, with more than 35,000 vaccinated with the Pfizer vaccine, while almost 2500 had received the AstraZeneca jab.

All quarantine workers had now been offered the vaccine, with a take-up rate of at least 92 per cent.

Over the next three weeks those who received the Pfizer vaccine will get their second dose, with the rollout to regional centres expanding this week.

The end of the state's 55-day COVID-free run on Sunday prompted the NSW opposition to renew calls for the government to ensure all hotel quarantine security guards were employed full-time so they did not work across multiple venues.

"Securing our quarantine hotels shouldn't be a part-time job. The role security guards play in hotel quarantine is too important," Labor health spokesman Ryan Park said.

"It's no good building a fortress if we end up leaving the back door open. Hotel quarantine is meant to be a bubble.

"It's not going to be effective if guards are forced to take other roles in other parts of Sydney to make ends meet."

NSW Health on Monday afternoon listed a number of new alert venues which were visited by the security guard, including Hurstville Aquatic Leisure Centre on the afternoon of March 10 and Hurstville Coles on the morning of March 13.

Anyone who visited those venues at those times should monitor for symptoms and get tested and isolate if they appear.