Three people in managed isolation at the Pullman Hotel caught Covid-19 in "separate events", authorities believe.

A report into what went wrong at the facility, in Auckland, has now been released after three returnees tested positive for Covid-19 last month after being let out into the community.

"Investigations into the cases are ongoing and it's possible we may never identify the exact cause of transmission but it's believed there were three separate events," Head of Managed Isolation and Quarantine Brigadier Jim Bliss.

"Genome sequencing confirmed all three cases had the variant first identified in South Africa and current research suggests that this and other new variants are more transmissible than previous variants.

"There is increasing evidence people can get infected if very small droplets remain suspended in the air. The risk of this sort of airborne transmission becomes higher in closed spaces with poor ventilation.

"The reports indicate that multiple factors contributed to the cases around the Pullman," he said.

The Pullman Hotel managed isolation facility reopened to returnees this morning after being deep-cleaned and other safeguards being put in place.

Bliss said the response included:

• changes have been made to the way the Pullman's corridor ventilation system is used, following an assessment of the ventilation system
  • corridor ventilation will operate 24 hours a day, as opposed to the two hours a day at the time of these incidents. This will significantly reduce the risk of airborne transmission
  • returnees will be advised to close all windows and trickle vents in their rooms before they open their door. This will encourage air to move from the corridor into the room, rather than having the air going out into the shared corridor
  • returnees will be limited in the times they are permitted to leave their rooms to set schedules. The upgrade of the Pullman's CCTV system, which is now complete, will help monitor compliance with these movement restrictions
  • lifts are considered one of the most high-risk common areas in the facility. Air filtration systems will be installed in the lifts over the next couple of weeks. In the meantime, the use of these lifts will be reduced to significantly mitigate this risk

"All of these measures are on top of already robust infection prevention and control measures which include physical distancing, the use of masks, hand sanitising and enhanced cleaning protocols in shared and high movement areas.