Water vapour, as one of the most prominent greenhouse gases, can effectively alter the radiative forcing and influence the global surface temperature once it enters the stratosphere. In January 2022, the underwater volcanic eruption Hunga Tonga-Hunga Ha'apai (hereafter Hunga) injected a huge amount of water vapour into the stratosphere and lower mesosphere, leading to an instantaneous increase in the global stratospheric water mass of 10%. This is the largest perturbation observed in the satellite era spanning more than 30 years. Despite its critical importance for accurately predicting the near-term climate impact, estimating how long the Hunga-introduced excess stratospheric water will persist has been challenging. We now have sufficient observations to characterise the decay of the Hunga water. For the first time, we are able to reduce the large uncertainty in estimation of Hunga-water residence time, by showing the decay of Hunga water using the daily near-global profiles and interpreting the changes using a model.