Green’s Dictionary of Slang

(Aus./N.Z. und.) a look, a survey, thus keep the peg, too keep watch.

1972		J. McNeill Old Familiar Juice (1973) 100: bulla: Better have a peg at the news, I s’pose.
1975		Bulletin (Sydney) 26 Apr. 45: We give it a peg and decide to give the Limp another try.
1986		R.G. Barrett Real Thing 62: Norton had a peg out of the side of his eye.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 137/2: peg n. 1 a look, a stare 2 (also pegger) a lookout. [...] keep (the) peg to act as a lookout, to keep watch.