Green’s Dictionary of Slang

1. (Aus./N.Z.) the buttocks; the anus.

c.1943		S .J. Baker Aus. Vulgarisms [t/s] 7: acre: The anus.
1965		J. O’Grady Aussie Eng. (1966) 9: Female ‘acres’ are generally referred to as ‘rears’ [...] one who is ‘six axe handles across the acre’ should [...] go on a diet.
1971		F.J. Hardy Outcasts of Foolgarah (1975) 24: Wiping between his toes and falling on his acre.
1993–2004		Probert Encyc. 🌐 Acre is slang for buttock.
2003		McGill Reed Dict. of N.Z. Sl.

2. the testicles [physical proximity to sense 1].

1993–2002

Probert Encyc. 🌐 Acre is slang for testicle.