In November 2021, the New Zealand Ministry of Education, in collaboration with the National Institute of Water and Atmospheric Research (NIWA) carried out a rapid observational study on ventilation and air quality in 18 classrooms at three New Zealand schools. This study confirmed that an efficient way of achieving good ventilation and reducing the transmission risk of COVID-19 is by opening doors and windows(natural ventilation). With winter approaching (June - August in southern hemisphere), there is a concern that fully opening windows and doors to achieve good ventilation may not be desirable or possible without excessive heat loss. Building on the approach and findings from the rapid ventilation study, a further study was carried out in a ‘unoccupied’, typical naturally ventilated New Zealand classroom (75m2 floor area) with simulated occupation. The aims were to assess how the ventilation was impacted by different opening areas, by differing indoor versus outdoor temperatures, and by supplementary measures such as portable air cleaners and fans. This was an urgent follow up study carried out over two weeks to inform the winter ventilation guidance to be provided by the Ministry to New Zealand schools. To similate occupation, a gas cylinder was used to release CO2 (gas tracer method), while smouldering incense sticks were used to generate aerosols. The decay in concentrations were observed as the gas and particles were removed from the space by the various ventilation methods being tested. Tests were then repeated on different days. From this, the actual and effective Air Changes Per Hour (ACH and eACH) were calculated for different window and door opening percentages, and for the supplementary measures. Summarily, the study findings indicate that typical, naturally ventilated New Zealand classrooms can achieve good ventilation through partial window openings and in some cases can be assisted by supplementary measures such as fans and air cleaners. Though noting the study was limited in its scope due to urgency, the tests conducted suggest that achieving this level of ventilation should not introduce thermal discomfort. The ‘uncocupied’ classroom windows had restrictiors, and if there were no restrictors, opening the windows wider could greatly exceed the ACH found in this experiment. The results affirm the findings of the previous study (NIWA, 2022), that natural ventilation provides a wide range of air change rates, and depending on the wind speed, this could be up to 29 ACH. Future studies should explore various ventilation measures with natural ventilation on the same day (ideally simultaneously in near-identical control and intervention rooms), as well as under the range of wind and thermal conditions over a typical winter school day. However, this study provides insightful findings and can in principle be transferred to similar situations in closed rooms that are occupied by more than a single person, such as conference rooms, waiting rooms and shared offices.