Green’s Dictionary of Slang

something or someone outstand, first-rate.

1905		Sun (Kalgoorlie) 15 Jan. 2/3: The classic language of some turf scribes! A contemporary heads a par ‘Betsy Burke a Bobby-Dazzler’! Why not ‘Best Man a Bontoshter,’ or ‘Rathsel a Ripping Snorter?’.
1906		Bulletin (Sydney) 22 Nov. 17/2: Occasionally one hears of a dabster (a variety of boshter) and a bon-toshter, which denotes something abnormal in the boshter line.
1907		Brisbane Courier 29 May 6/3: A ‘bonzer’ kick at football is a long, well-directed, truly-judged kick. Perhaps ‘bontoshter’ may be taken as conveying a more complete admiration than the others.
1914		Sun. Times (Perth) 11 jan. 3/2: Tuesday evening saw a duststorm – a beauty, a regular bontoshter – and that gave us a cool night.
1915		Sun (Kalgoorlie) 6 June 5/8: Don’t miss the Belgian Band – its a bonzerino and a bon toshter.
1917		Sun. Times (Perth) 18 Mar. 8S/7: A Brobdingnagian bug was he, / He nearly knocked me blind – / A regular bontoshter of / The Thysanarian kind.
1918		Balonne Beacon (St George, Qld) 27 July 2/5: [A]ccording to track work some of the horses are real ‘bontoshta’s’ – they gallop a long way in a long time.
1931		Mail (Adelaide) 30 May 9/5: ‘Boshter’ and ‘bonza,’ meaning good, with the superlative ‘bontoshter,’ are not heard as frequently as of yore.
1934		Sydney Sportsman 29 Sept.12/2: [of a dog] Arthur Ell, well-known to Coaly City greyhound patrons has had bad luck. Tyrone Boy, bon-tosher of his kennels, broke down at Newcastle last week-end.
1938		Nthn Miner (Charters Towers, Qld) 5 Sept. 4/3: A fair and square ‘bontoshta,’ and a man I’m proud to know / Is this more than brilliant unit in the ranks of Cobb and Co.