Green’s Dictionary of Slang

1. excellent, first-rate.

1905		Aus. Town & Country Jrnl (Sydney) 31 May 38/2: After picking himself up [...] he said, ‘I call it real boscar living up here; there is a bonster cellar round at the back of the house; Barney and me found it.’.
1908		Marlborough Exp. 16 Jan. 6: ‘What ho, she bumps,’ ‘bonster,’ and its corruption ‘boshter’ and ‘bosker,’ had a great run some little time ago, but are seldom heard nowadays.
1910		Dubbo Liberal (NSW) 2 Nov. 3/1: Mr. Bullock has some bonster bronzewing turkeys.
1950		Bulletin (Sydney) 29 Mar. 10/3: Slang words do drop into disuse [...] for instance, I can’t remember when I last heard any thing praised as ‘bonzer,’‘bosker’ or ‘bonster’.

2. substantial.

1912		Mt Alexander Mail (Vic.) 12 June 4/2: Hapless Billy screamed with fear, / As the monster seized a bonster / Swinish hold on Billy’s ear.
1940		Waikato Indep. 4 Oct. 1: [advert] Good Supper and a ‘Bonster’ Supper.