In 1967 Warwick Brock first used the term at the 3rd National Folk Festival in Wellington, NZ, at the end of the final concert. It was an unrehearsed, all performers on stage to close the proceedings, not quite up to standard performance. Pretty ugly really!
But we loved it.
And ever afterwards Frank Fife would use the term at the end of an evening, as an open invitation for everybody to join in.
I don't know how/when it migrated to Oz - quite a few of our folkies have migrated there over the years.
Robyn