Green’s Dictionary of Slang

peg n.⁶

it’s a peg [ety. unknown]

(Aus.) a phr. used to indicate pleasure, that’s wonderful, excellent etc.

1945		Baker Aus. Lang.
1955		N. Pulliam I Travelled a Lonely Land (1957) 237/1: peg – something exceptionally fine.

strip a peg in Plunket Street (v.)

see under strip v.