Green’s Dictionary of Slang

(Aus.) a positive or cheerful state, condition, mood.

1888		‘Rolf Boldrewood’ Robbery Under Arms (1922) 177: The horses were in a great buckle, and carried us over to Bargo easy enough before dark.
1897		Star (Canterbury) 24 Apr. 2/6: I was in pretty fair buckle when I landed at Port Melbourne.
1902		Bulletin (Sydney) 5 Jul. 16/2: Then the mob were spelled out for two days at Morgan, and fed on hay to put them in buckle for the rail trip.