These relic buildings were constructed in (around) 1943 and have remnant insulating materials on them still. I have gathered samples of three types - all of which look like they were applied in-situ in 50mm thick blocks glued (with mortar) troweled into place and then over-plastered, and with faux "block lines" too - you can see these lines in this photo - under the "windows" which are actually access spaces with a floor trapdoor allowing a view down into the retting tanks which of course are full to the roof and sealed and otherwise inaccessible.
I have yet to get hold of the Ministry of Works drawings and specs, but there is (a) granulated cork (b) rock wool and a third material (c) a very light-weight pale grey foamed material that is just like Hokey-pokey. This is the material used on all insulated faces of this Fairlie building
It is not "natural" and is definitely a foamed low density cement very fine sand mixture quite crumbly and fragile but a very effective insulant. It is obvious that experimentation was going on. What looks like concrete block mortar lines is the glue pattern for the insulant - the building walls are concrete. Note that the insulated sections were set-back 50mm so that after insulating and over-plastering the whole wall - including the structural elements around the doors and windows - ended up flat - no "junk job" these mills - well made and well designed and only the serendipity of the after-war recovery of Europe's linen flax industry stood between these mills and another lucrative pastoral agricultural enterprise for NZ.