Thread:Suggest creating a category structure (3)

I would lean toward skipping what you've called 1st level and 2nd level. If something is in level 3 (or OERNZ_Something_Not_Mentioned) then it can be found with the correct search. (I would hope the community someday supports installation of the MWSearch (Lucene) extension.) I don't see the value of having a giant OERNZ category whose contents can be determined in other automated ways.

I also expect there should be categories (or ideally tags) for NZ curriculum units.

But I'm also very skeptical of being too prescriptive at the start. I believe the sort of people that like using wikis tend to be more self organizing. This is why I've argued for a tagging system which would associate a tag cloud with each page, with visitors able to see the top ways users have tagged the page.