Green’s Dictionary of Slang

(Aus.) a lavatory, esp. a communal urinal used by schoolboys, soldiers etc.

1865		Leaves from Diary of Celebrated Burglar 154/1: I ‘officed’ Folkstone to ‘cheese it’ while I went to the ‘Dyke’ to turn them [i.e. stolen wallets] out.
1923		J. Manchon Le Slang 104: Dike, les cabinets; to do a dike, aller aux lieux, aux gogues.
1940		M. Marples Public School Sl. 112: Other synonyms [for ‘lavatories’] are rears, lats [...] dykes (Oundle, 1920 +), [etc.].
1960		N. Hilliard Maori Girl 74: The dyke’s in the bathroom, and someone’s sure to stink the place out.
1960		‘Nino Culotta’ Cop This Lot 28: Been ter the dyke yet? [...] Like the bloody Hotel Australia, ut is. Everything laid on except sheilas.
1978		N. Hilliard Glory and The Dream 238: And uses the toilet too, [...] off it goes straight to the dyke.
1986		T. Winton That Eye, The Sky 120: It’s normal for you to go to the dyke fifty times a night.
1993		M.B. ‘Chopper’ Read How to Shoot Friends 61: Believe it or not, we couldn’t even find the toilets. ‘This is insanity,’ said Dave. ‘We can’t even find the dyke, let alone the bloody poofs.’.
2003		McGill Reed Dict. of N.Z. Sl. 71: dyke A toilet, often the outdoor variety whose bottom is not in sight.