A number of small area geographies are used in Australia to investigate primary care relevant outcomes/behaviours and to manage the supply of Primary Care Providers (PCP) that influence these outcomes. However, very little research exists on the choice of a small area geography suitable for these purposes. We evaluated a large basket of Australian small area geographies to determine which geography is optimal for investigating relationships between PCP supply and the use of PCP services. We used linked data to evaluate the relationship between PCP supply and the likelihood of a patient visiting a PCP, after adjusting for individual level covariates. PCP supply was measured at different geographies including Local Government Areas (LGAs), Primary Health Networks (PHNs), Statistical Areas-1/2/3 and Remoteness Areas. Overall, the strongest relationships between PCP density and PCP use were found when LGAs were used to measure PCP density. Large geographies such as PHNs also detected strong relationships while custom built geographies such as Primary Care Service Areas were not significantly better than the rest. Existing geographies such as LGAs may be suitable for investigating the effect of PCP supply at state or national scales.