To clarify: In Australia, when we say "Bush" we mean forest/any area that isn't all urbanised. It does not actually refer to bushes/shrubbery. So when I say "in the bush" I mean "On the ground in a forested/vaguely natural area". Generally there'll be a river somewhere, but leeches go wherever there's dampness...

According to my google-fu, "Land leeches are common on the ground or in low foliage in wet rain forests. In drier forests they may be found on the ground in seepage moistened places. Most do not enter water and cannot swim, but can survive periods of immersion."

http://australianmuseum.net.au/Leeches