Green’s Dictionary of Slang

(Aus.) something excellent; also as adj.

1903		North Western Advocate (Tas.) 12 Mar. 2/8: I must especially mention Harry Gale, he taking three catches, two of them beauties, and one of them a regular ‘swinjer’.
1942		Le Courrier Australien (Sydney) 5 June 7/1: For notable persons or things you use many terms like [...] whopcacker, snodger, snozzler, swinjer [etc].
1955		N. Pulliam I Travelled a Lonely Land (1957) 240/1: swinjer (swiz) – something exceptionally good.