Well, no, not montheism. There are many Wanjina, not just one. The Wanjina are not something that people worship or pray to either, Wanjina are just there, they simply exist, and were there before humans.
Its also interesting to note, that many different tribal groups have their own Wanjina, that live in their country.
The Rainbow Serpent, (who has many names), is the only common belief among many groups, but not all.