Found at the edge of a stream. Sheath-forming iron- and manganese-depositing bacteria possibly in the Sphaerotilus-Leptothrix group. Individual tubes to >640µm long and 1.3µm wide. To take the id any further requires some serious laboratory equipment and skills. For a recent reference to this group see Schmidt et al. 2014, FEMS Microbial Ecology 90: 454-466.
Stephen Skinner of the NSW Herbarium said "I concur that it is bacterial...and there is more in the notes than I could have said. As far as I am aware, these iron bacterial patches are not any cause for worry, but in New Zealand all such things require to be reported and checked!