Rowan, in the case of the Australian languages you mention, their individual status as "languages" or "dialects" is presumably based mostly on "mutual unintelligibility." If there's never been an established written tradition among native speakers, it would be pointless to demand one if the "speech communities" are separated rather than united by their linguistic systems.

My guess is, too, that there's a certain inevitable amount of fudging in the analysis into languages and dialects, with borderline or uncertain cases more or less arbitrarily consigned to one category or the other.

Danish amd Norwegian are mutually intelligible, as are Scottish Gaelic and Irish, but they're customarily regarded as separate languages. Is Galician a dialect of Spanish ? Like Scots, it has a tradition of literary but not, I think, scientific publications. Some rural Sicilian dialects are said to be unintelligible to northern Italians (and vice versa). Are they separate languages ?

My point is that in many cases, like English and Gaelic, there's no doubt they're separate. In many others, like English and "broad" Scots, there's no entirely objective way of deciding.