I've always struggled to make large numbers of trees. Well, more correctly I've never really attempted it, unless you count using sphagnum moss spray painted green 25 odd years ago.
In my wanderings on the net I came across this last night. While not New Zealand I think its an idea worth looking at.
The base of the forest is expanding foam filler, that is shaped, painted and flocked. Looks a lot faster and cheaper than using Woodland scenics foliage blocks.