The current study investigated the assessment of depression, anxiety, and stress during normal and COVID-19 pandemic conditions. Generalisability theory (G-theory) was applied to examine stable and dynamic aspects of psychological distress and the overall reliability of the Depression, Anxiety and Stress Scales (DASS-21), using data from two independent samples collected on three occasions with 2- to 4-week intervals. The US data (n = 115) was collected before the COVID-19 pandemic, and the New Zealand (NZ) data (n = 114) was obtained during the pandemic. The total DASS-21 demonstrated excellent reliability in measuring enduring symptoms of psychological distress (G = .94-.96) across both samples. While all DASS-21 subscales demonstrated good reliability with the pre-pandemic US sample, the subscales' reliability was below an acceptable level for the NZ sample. Findings from this study indicate that the overall psychological distress is enduring and can be reliably measured by the DASS-21 across different conditions and populations, while shifts across depression, anxiety and stress levels are likely during emergency and uncertainty, as seen in the COVID-19 pandemic.