I wouldn't do this with htaccess. I would create canonical names (cname) records with your domain registrar to do this. Makes things much handier on you. Who are your domain names registered with? If i'm familiar with them I may be able to provide you with instructions or a link to some instructions.
Your logic seems to be flawed. You are rewriting www2.mysite.org.nz to test.mysite.org.nz. What you should be doing is rewriting test to www2. A cname is still the best way.
“The best thing about a boolean is even if you are wrong, you are only off by a bit.”