Green’s Dictionary of Slang

1. a tramp.

1898		E.E. Morris Austral Eng. 333/1: Overlander, n. [...] (2) A slang name for a Sundowner.
1909		E. Waltham Life and Labour in Aus. 57: From this point the responsibilities of the ‘Overlander Boss’ begin. [...] The ‘Boss,’ who takes the contract to deliver the cattle.
1941		K. Tennant Battlers 94: I bought a thousand head o’ cattle, and then turned Overlander.

2. a large mosquito.

c.1887

K.G. Gallop Never Never Land n.p.: Huge mosquitoes known here as ‘Scotch Greys’ or ‘Over-landers’ added their quota of misery [AND].