Regarding 'collectors' motives', I agree that finding a treasury of Child ballads in the 1950s might well have been unlikely anyway, but the full quote from McKenry is worth posting:

"...Jones established a practice which would be repeated a decade or more later by the next generation of field collectors – John Manifold, Russel Ward, Nancy Keesing and John Meredith – that of restricting their collecting to songs unique to Australian life. It was a practice which was to cost Australia dearly, for only a minority of singers' repertoires concerned such matters and so were recorded. Happily, that practice became untenable in 1956, when Meredith first recorded Sally Sloane. Sally's inherited repertoire of traditional Irish and other songs was just too wonderful to ignore, but by then the damage was done. In the fifteen years following Jones's first field forays in 1940–41 a generation of informants was lost, and unknown musical treasures with them."

If you look at the song lists on the page linked by the Guest above, the only British-origin songs are those dealing with transportation, plus of course 'The Wild Rover', whose exploits appear to have qualified him as an honorary Aussie.