Green’s Dictionary of Slang

something or someone excellent.

1904		Bulletin (Sydney) 14 Jan. 27/4: My interpretation of boshter is a splendid thing, or person. It is sometimes an adjective [...] It is compared in the Australian Slanguage Text-Book thusly: Boshter, splendid; Bonzah, more splendid; Bontostah, most splendid.
1910		Dly Herald (Adelaide) 20 Aug. 15/3: ‘Ah me! I’m his bonzer tart! Ah me! Ah me!’‘You are! You are! My bontoshter bit of fluff!’.
1910		Bunbury Herald (WA) 5 Nov. 4/3: The merits of the programme submitted was summed up by the genial Canon in a very few words. It was, he said, at least ‘bonsa,’ if not ‘bontosta,’ possibly ‘bontosterino’.
1916		Avon Gaz. & York Times (WA) 1 Apr. 2/6: [I]t was a bon-toshta procession.
1922		West Australian (Perth) 18 Aug. 4/1: [advert] BUY BONTOSHTER BARGAINS FROM DAVEY, THE SHOEMAN.
1966		C. MacInnes All Day Saturday 105: ‘If he does – bontoshter. If he doesn’t – well, no little Nancy for Mr Julius Macnamara’.