Key questions remain unanswered after NSW Health wrapped up investigations into two unrelated coronavirus transmission events at Sydney quarantine hotels.

The spread of the virus within the Mercure and Adina hotels earlier this month highlighted the increased risk of transmission as new strains of coronavirus hit Australian shores.

NSW chief health officer Kerry Chant said while it remained unclear exactly how the virus managed to spread between guests in adjacent rooms in the Adina hotel in early April, officials have developed theories for how it could have happened.

“The transmission could have occurred through aerosols down-drifting between the two rooms or via a shared partitioned balcony,” Dr Chant said.

Aerosols is another word for airborne particles.

At the Mercure Hotel, there was no obvious method of transmission found.

“There wasn’t – based on engineering and other assessment – an aerosol transmission route,” Dr Chant said.

“But obviously, we know with COVID transmission that it can sometimes be very subtle in the way it’s occurred.”

At the Adina Apartments Hotel near Town Hall, seven people from two different family groups who arrived from different countries and on different days were found to have been infected with the virus.

They stayed in adjacent rooms in the hotel. Three of the cases were reclassified from acquired overseas to locally infected as the investigation progressed.

At the Mercure Hotel in Sydney’s CBD, a returned traveller who stayed in a room next to where two members of the same family were housed was reclassified as a local case on Tuesday this week.

“An investigation into how COVID-19 transmission occurred between returned travellers who had been in quarantine at the Mercure Hotel has led NSW Health to reclassify one case, previously reported as overseas acquired, as locally acquired,” NSW Health wrote in a statement on Tuesday morning.

All three Mercure Hotel cases had the highly transmissible South African strain of coronavirus.

There has been an increasing prevalence of mutant strains of coronavirus, labelled “variants of concern” by NSW authorities.

In the latest weekly COVID-19 report, the health department wrote that 41 per cent of overseas-acquired cases were diagnosed with one of three variants of concern that originated in the UK, South Africa and Brazil respectively.

Between November 29 last year and last Thursday when the report was issued, 584 returned travellers had been diagnosed with coronavirus.

Of those, 123 people, or 23 per cent, had a variant of concern.

There were no new local cases on Wednesday but nine new cases in overseas travellers.