Its worth checking out whether they are looking for a one off trip or an ongoing committment, as it varies.  The one we did in Kaimanawas for TNHS was a onecer, and hey will probably be doing the same thing this summer.  Also DoC has heaps of volunteer work from November to March, usually a week at a time.  I can definitely recommend that.

But, if you want to see a Blue Duck just head to one of the Ruahine rivers - Apias Ck is very good, or even better up a West Coast river!!