Estimation of the degree of undercount is important for disease surveillance. Capture–recapture techniques are now being used to evaluate the completeness of disease ascertainment. This study estimated the level of under-notification of giardiasis in the Auckland adult population using a capture–recapture method. Two independent datasets of giardiasis cases [ges ]15 years were generated from the 1998–1999 Auckland Giardiasis Study (AGS) case database and cases notified to Auckland Regional Public Health Services (ARPHS) for the same period of time. Cases were matched and under-notification was estimated using a two-sample capture–recapture method. During the 12-month period, 199 cases participated in the AGS and 413 cases were notified to ARPHS. The capture–recapture calculation indicated that only 49% of cases were notified. Under-notification by a factor of 2 obscures the true burden of giardiasis. Socio-economic conditions and water quality may influence disease notification inversely. Capture–recapture techniques are useful in evaluating the completeness of surveillance.