Back when a higher proportion of people were ready to accept buses as a way of getting around, a smaller catchment population would generate sufficient users to justify the service. Suburban Australians are more transit-oriented than Americans, so the same development densities in the two countries will yield different levels of transit usage. Local ordinances in the US are now requiring developers to aim for densities far higher than those in Australia's bus-connected suburbs. This does not seem to be a very stable parameter for planning. In other words, there is in absolute terms no 'minimum density' to support a public transport service.
Services are only more or less used and subsidised.
To specify a 'minimum density' requires us also to state assumptions about local form and structure, propensity to chose public transport, and the level of subsidy adopted. Undoubtedly many who are eager for change in our cities regard quantitative niceties as being irrelevant, stressing the need for vision more than knowledge. What we have here is a popular belief which is promoted on the basis of an analysis of empirical data . It must at least stand up to scrutiny on that score, and it fails to. The technical point is important for one very real reason.