Health authorities in NSW have warned several people who could have been exposed to COVID-19 inside a Sydney quarantine hotel have already travelled interstate.

Investigations are underway after three returned travellers from two families staying at the Mercure Hotel in Sydney’s CBD tested positive to the South African variant of the virus.

The families were staying on the 10th floor at the hotel in adjacent rooms and NSW chief medical officer Kerry Chant said 40 other return travellers were staying on the same level.

“We have managed to contact 36 of those individuals, a number have gone into other states and territories and those states and territories have been alerted,” she said.

“We are urgently escalating contact with the remaining four.”

The infected travellers sat in the same section of the plane that arrived in Sydney from Malaysia earlier in April, but tested negative for COVID-19 on day two of their stay in hotel quarantine.

Dr Chant said it was unlikely the group was infected on the plane.

Investigations are continuing as to how the virus could have spread inside the hotel.

“We don’t have a definitive conclusion about the way transmission occurred,” Dr Chant said.

“The question is, could other people have been exposed?”

The infected travellers have been transferred to the NSW government’s “health hotel”, where they will remain until no longer infectious.

NSW Premier Gladys Berejiklian said the state’s hotel quarantine program had been a success.

“Without being too immodest, our quarantine system has really stood the test of time and the challenges on it every day – 3000 Aussies coming home every week through Sydney Airport, more than 5000 Aussies in hotel quarantine in Sydney at any given time is a huge job,” she said.

Earlier this week three people in one family acquired the virus from another family staying in an adjacent room in the Adina Apartment Hotel Town Hall in Sydney’s CBD.

The Premier has rebuffed the Western Australian Premier’s calls to temporarily ban return travellers from India, following two COVID-19 infections in a Perth hotel quarantine this week.

Ms Berejiklian said no one country should be targeted and ruled out.

“I don’t think it is fair or appropriate to distinguish one nation over others,” she said.

“Things change, the rates of infections go up and down across the world.

“Aussies who want to come home should have the right to do that.”

The NSW mass vaccination centre at Sydney Olympic Park. Photo: ABC

On Thursday, Ms Berejiklian visited the mass vaccination hub at Sydney’s Olympic Park, which will be the only place in NSW where both the AstraZeneca and Pfizer vaccines can be administered.

Deputy secretary of NSW Health Susan Pearce said the centre would be ready to open in mid-May when more Pfizer vaccines will arrive in Sydney.

“We need this centre if we are to deliver the number of vaccines that we need to [and] to compliment the GP network,” she said.

The centre will be able to vaccinate about 30,000 people each week but the Premier said this could rise once “we get our rhythm”.

NSW had seven new cases in returned travellers in the 24 hours to 8pm Wednesday, from 10,264 tests undertaken.