Green’s Dictionary of Slang

something or some considered excellent, first-rate.

1901		Gundagai Indep. (NSW) 25 May 3/1: It will be interesting to notice how the expatriated Gundagai gelding will shape beyant the border after his enforced spell. A Gundagai trainer votes him a regular ‘bonster’ over 6 furlongs.
1904		Bulletin (Sydney) 14 Apr. 29/1: Re that bulwark of Austral Slanguage — ‘Bonster’. .. A bonser or bonster is comparatively superior to a bons.
1905		Aus. Town &Country Jrnl (Sydney) 24 May 28/3: ‘My missus is a bonster for sendin’ me curries. Hot stoo is hard to tackle when yer on the down grades’.
1907		Sydney Sportsman 5 June 1/3: Blue Streak was boss of the situation at any part of the trip, and coming at such a bonster as Superstition in the last couple of furlongs, he settled him fair dinkum.