The Pullman Hotel managed isolation facility in Auckland.
Photo: RNZ / Dan Cook
The Auckland facility was closed after three guests tested positive for the virus in separate incidents in January, then operated at a 50 percent capacity.
An independent report into the facility was unable to conclude with absolute certainty how transmission occurred, but it was likely to be airborne.
Transmission in at least two cases was most likely through exposure to aerosols in a lift or in relation to a lift lobby; both shared spaces that were confined, congested and poorly ventilated, the report said.
In one case, a returnee s room appeared to have been the most likely location of aerosol transmission due to the particular orientation of the door of this room in relation to the lift lobby.
Grand Millenium MIQ facility in Auckland.
Photo: RNZ / Dan Cook
It comes as an investigation begins into how Covid-19 was transmitted in those facilities.
Genome sequencing showed the virus spread between two returnees at the Grand Mercure, who arrived back on different flights, days apart.
At the Grand Millenium, a returnee, a cleaner and two security guards are in a single chain of transmission.
There are 224 isolating returnees across both hotels and once they all complete MIQ there will be a review of potential sources of spread at the facilities including the ventilation systems.
Bliss told
Checkpoint risks presented by the ventilation systems had always been assessed by health experts as being very low.
Photo: RNZ /Dom Thomas
One year ago today, all arrivals at New Zealand airports began to be taken to managed isolation hotels for a 14-day stay.
Since then, 130,600 people have passed through the 32 border hotels.
Joint head Megan Main said staff working at the facilities had made considerable personal sacrifices to keep those people safe.
More than 850 people have tested positive for Covid-19 during their stay - including about 95 current cases.
Main said MIQ had been a process of continuous improvement and the biggest learning curve had been adapting at pace . If something goes wrong, we get together and learn from it. If our operating environment needs changing, we work together with colleagues from the Ministry of Health in particular to look at new policies we need to introduce. Anything we need to do to strengthen infection prevention control. It s faced paced, she said.