Green’s Dictionary of Slang

bonster n.

something or some considered excellent, first-rate.

[Aus]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.
[Aus]Bulletin (Sydney) 14 Apr. 29/1: Re that bulwark of Austral Slanguage — ‘Bonster’. .. A bonser or bonster is comparatively superior to a bons.
[Aus]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’.
[Aus]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.

In derivatives

bonsterina (n.)

the female version of sense 1.

[Aus]Bulletin (Sydney) 14 Apr. 29: A bontosher is a real slasher... A bonsterina a female bonster.
[Aus]Grafton Argus & Clarence River General Advertiser (NSW) 30 Dec. 4/5: Thursday, in the language of immortal Billy Shakespeare, was a ‘bonzer’ – yea, a ‘bonsterina.’ And the storm that night!