My impression is that once you live in an area for a while and still have to solicit for random strangers to share your home it is a possible warning sign.
This is definitely not true. Many people share for a large number of reasons. The success of the internet home sharing site AirBnB would not have happened if there were not a number of possible positive experiences to be had. AirBnB offers short term accommodation sharing peoples homes. Many people love staying in other peoples places. And lots of people love having other stay.
Of course, as JandM says, you need to check out the person advertising. You may be lucky and find someone great who will become your connection to a wider New Zealand community as they introduce to their friends.