Just had an interesting conversation on the phone with one John Archer, who contacted me because he'd seen an old war poem on my site. It turns out that the version I posted, which I'd got from old papers belonging to my mother, is just one of many, and a quite young version as well.
The songs tend to have a dug-out as a link, but not always. In the earlier versions there are shanties and the like, rather than dug-outs. It's amazing how folk/popular songs tend to keep alive under a variety of different disguises.
John's posted a draft page on his site, so I'm not sure whether it's there for public display yet. However, it gives a number of different versions of the poem/song, and shows how its roots lay in old Negro songs from the Civil War period.
John's site is well worth visiting. He's obviously put a lot of effort into collecting songs, particularly New Zealand ones, and being a work in progress, it's going to be a place to keep on visiting.