Green’s Dictionary of Slang

(Aus.) sexual intercourse.

1963–74		B. Humphries Barry McKenzie [comic strip] in Complete Barry McKenzie (1988) 105: If we weren’t on sacred ground, I’d give youse a ‘wellington’ on the spot.
1983		R. Aven-Bray Ridgey-Didge Oz Jack Lang 52: Wellington Boot Root.
1989		R.G. Barratt ‘Wellington’s On the Other Foot’ in What Do You Reckon (1997) [ebook] What if my mate John had gone round to Miss X’s house demanding a Wellington.
1999		G. Seal Lingo 91: Some other examples: [...] new south for mouth, warwick farm for arm (New South Wales) and wellington boot for root (as in fornicate).