There is a large body of existing count data in the literature for penguins at their breeding sites in Antarctica and the sub-Antarctic islands. These data have been summarised into several compilation documents and their accuracy assessed therein on a site-by-site basis. There has been no previous attempt to determine how the accuracy of regional scale abundance estimates derived from these data might be assessed. A formal abundance estimator appropriate to regional scale abundance estimation from existing data is developed and the assumptions underlying that estimator outlined. Existing data for Adelie penguins are then examined to determine how well the estimator assumptions are fulfilled, and hence how accurate regional estimates of breeding populations, as derived from existing data, might be.