Green’s Dictionary of Slang

acre n.

[SE acre, a large area + play on SE ache-r]

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

[Aus]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.
[Aus]F.J. Hardy Outcasts of Foolgarah (1975) 24: Wiping between his toes and falling on his acre.
Probert Encyc. [Internet] Acre is slang for buttock.
[NZ]McGill Reed Dict. of N.Z. Sl.

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

Probert Encyc. [Internet] Acre is slang for testicle.