Green’s Dictionary of Slang

of quality, excellently, very well; of size, substantially.

1913		Dungog Chron. (NSW) 10 Oct. 7/3: ‘Yea I even will I give up to her my own chamber, and as things are going bosker, to-night we will have much rejoicing and high jinks’.
1913		Sydney Stock & Station Jrnl 21 Oct. 3/4: It is a bosker big rock, and when you get to the foot of it you feel dreadfully small and insignificant.
1946		Northern Champion (Taree, NSW) 16 Jan. 1/5: It’s a bosker big river, the Manning, / Have you ever been on it, my boy?