Green’s Dictionary of Slang

1. (Aus.) a derog. term for any non-white person [= wog n.¹ (1)].

1967		‘Whisper All Aussie Dict.’ in Kings Cross Whisper (Sydney) xliii 11/3: wogs: Anyone not Australian. Also hollow logs.
1999		G. Seal Lingo 91: A hollow log is a wog.

2. (Aus./N.Z. prison) a (racing) dog; a drug detection dog.

1983		R. Aven-Bray Ridgey-Didge Oz Jack Lang 11: G’day, I was here first. That’s my hollow log sitting there.
1988		McGill Dict. of Kiwi Sl. 57/1: hollow log dog; rhyming slang, among shearers.
1991		R.G. Barratt ‘What Are You Laughing At?’ in What Do You Reckon (1997) [ebook] She was a grouse hollow log, a cross bull terrier-kelpie.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 89/1: hollow log n. a dog, esp. the narcotics detection dog.
2003		McGill Reed Dict. of N.Z. Sl. [as cit. 1988].

3. (Aus. prison) an informer [= dog n.² (6b)].

1990

Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Hollow log. An informer. Rhyming slang for ‘dog’.