An investigation in the Pullman Hotel outbreak is looking at whether airborne particles generated by bowel movements could have been a factor.
A Ministry of Health report into the Pullman Hotel outbreak last month, where guests from the hotel were found to have Covid-19 after they had left the managed isolation facility.
It recommended an investigation of bathroom exhaust fans because airborne virus particles can be generated by bowel movements of infected people.
Further understanding of whether that played a part was needed, it said.
That included checking fans to see if they complied with the Building Code's fresh air standard.
Ventilation has long been one of the suspected spreaders of the virus between hotel guests but the report said the cause of the Pullman outbreak may be because of multiple factors and may never be known.
Evidence was mounting that small particles of the virus could stay airborne for a long time and travel relatively long distances, the report said.
A number of changes had already been made to the hotel, such as ventilating corridors and requiring guests to open windows in certain circumstances.
But lift ventilation remained a concern and upgrades that were needed were yet to be done, the report said.
Human behaviour remained a key risk, including unnecessary and unsupervised movement around the hotel.
"The more time an [unknown] infected returnee spends in shared spaces outside their room (eg in the lift), the greater the risk that aerosolised viral particles may remain suspended in the air of that indoor space," it said.
The report revealed the Ministry of Business Innovation and Employment said the hotel, which was closed for investigation, had to be opened again by 15 February because it was needed for returnees with MIQ vouchers.
It did reopen but at half capacity to reduce risk and for investigations to continue.