Most states and territories in Australia have adopted the Problem Gambling Severity Index (PGSI) of the Canadian Problem Gambling Index as the standard measure of problem gambling in their prevalence studies and research programs. However, notwithstanding this attempted standardisation, differences in sampling and recruitment methodologies and in some cases the modification of the scoring methods used in the PGSI have lead to substantial difficulties in comparison of the prevalence rates obtained in different studies. This paper focuses on how these two actions may significantly underestimate the true prevalence percent of problem gambling in Australian studies of the prevalence of problem gambling. It is recommended that the original and validated version of the PGSI is used in future Australian prevalence studies and that prevalence in community studies is studied across the whole community not arbitrarily restricted sub-samples. The adoption of valid scoring methods and unbiased sampling procedures will lead to more accurate and comparable prevalence studies.