Green’s Dictionary of Slang

long tail n.2

[ety. unknown; ? the slow progress of treacle off the spoon]

(Aus.) treacle.

[Aus]Baker Popular Dict. Aus. Sl.
[Aus]Baker Aus. Lang. 81: Consider, for example, these slang names for treacle and golden syrup: longtail, spare boy, Kidman’s blood mixture, Kidman’s joy, beetle bait, black jack, bullocky’s joy, cocky’s joy and tear-arse.