The role of atmospheric rivers (ARs) for extreme ablation and snowfall is examined at Brewster Glacier in the Southern Alps, the site of the longest glacier mass balance record in New Zealand. By global standards, New Zealand is strongly impacted by ARs. Here it is shown for the first time (in New Zealand) that ARs strongly contribute to extreme snowfall and ablation and thus mass balance overall. Vertically integrated water vapor transport (IVT) exceeds 1,600 (800) kg·m−1·s−1 for the largest ablation (snowfall) events, marking these as very strong ARs. The proximity to the freezing threshold during extreme snowfall events indicates the sensitivity of mass balance to temperature variation. Importantly, similarly high rates of IVT during some extreme ablation and snowfall events also occur outside of conventional AR spatial structures. This finding indicates that AR detection algorithms may substantially underestimate the importance of extreme IVT for New Zealand and elsewhere.