It is not set down as a requirement anywhere. The rules are here.
http://www.immigration.govt.nz/migra...isit/visitors/ For a visitor visa to be issued, INZ need to know that the person is going to spend a certain limited time in NZ, then go back to their own country, and that they will have the means to support themself during their holiday. Therefore, a return air ticket is required, and they can ask to see proof of finances, and also that the person has a home/job/family/study course or whatever to go back to at a certain time. As you will see, the requirement for funds is less if someone has sponsored the visitor, but this isn't a matter of just a letter. I suppose a letter inviting someone to stay for just a limited amount of time might be an extra suggestion of when that person will leave... ?