The change from access=no to access=private was intended to solve a routing issue. We were attempting to route from the site to the Tasman Highway, but Valhalla was using inappropriate roads/tracks. Investigation showed all the internal roads with access=no were preventing any routing. 

If the permissions need to be relaxed further, that's fine. We just need routing to be possible at all.

That the site is no longer a mill with port facilities makes our quest less relevant - thanks for the useful weblink!