Green’s Dictionary of Slang

(Aus./N.Z.) a general intensive, describing something extreme or large.

1941		Baker N.Z. Sl.
1943		J.A.W. Bennett ‘Eng. as it is Spoken in N.Z.’ in AS XVIII:2 Apr. 90: Equally meaningless, though of older antecedents, are [...] the intensives hanguva, hangershun – modified forms of helluva.
1945		Baker Aus. Lang.
1978		‘A.P. Gaskell’ All Part of the Game 191: The more acceptable variations [of hellashin] were ‘hangashin’ and ‘heckashin’.
1984		Partridge DSUE (8th edn) 528: [...] since ca. 1920.