text: I just split the reserve near Manuka.
  Don't be scared by multipolygons, see this one as an example: https://www.openstreetmap.org/relation/3570376 it's massive but very useful. Multipolygons allow you to use the same line in different ways (for different objects), see this one https://www.openstreetmap.org/way/556694955 , which allowed me to split the Whiteford Res. in pretty much 30 seconds.
  The most significant example for this would be a coastline https://www.openstreetmap.org/way/179751300, this one is used as a border for the sea polygon and at the same time as a border for the land area. Without using a multipolygon relation, it'd have required drawing this huge coastline twice, which means loosing precious time. Usually when two complex polygons share a line, you can map them using a multipolygon relation.
  In our case, the reserve shares its borders with a residential area and with the see, we might have been able to map it out without even drawing a single line if we would have used a multipolygon.
  In case you are interested you could have a look here https://wiki.openstreetmap.org/wiki/Relation:multipolygon
  Years ago I started small with buildings with holes inside their perimeter (example https://www.openstreetmap.org/relation/7641592 ) and eventually decided to go to NZ to bother other mappers with relations stuff :D
  Yeah, you explained yourself pretty good, it's me the non native english which might find it hard to express himself :)
  I'm not sure if we should add the natural=wood to all of those three reserves, I'm going to give a read at the natural reserve wiki page, in order to see if they say something about combining it with the natural tag.
  Thanks again, cheers!