Sometimes, of course, this is all left to chance. In these instances, it is not uncommon for one or two dominant players to either kill a session or build it into a tour-de-force. I know because I have done both in my time.
My travels in Ireland, Scotland and Europe have revealed a different approach to the session in many bars. One or two key musicians (usually singer-guitarists) are paid, either with money or free beer, to "make" a session. They sit at a table, invite a few friends and play songs and tunes to seed the session. There may well be a few of these in New Zealand too, but I've not found them. There's always something faux about these sessions, but the singing is often good and occasionally there is a standout musician. It's also a place where visiting musicians can engage - which may, in turn, lead to an invitation to the secret session of the old hands elsewhere in the village. If not, it's best just to assume there isn't one.
There a list of open session in New Zealand here.