The government's website IS clear.
You can join KiwiSaver if you're:
a New Zealand citizen, or entitled to live in New Zealand indefinitely
Once you have Residence you are entitled to stay and work in NZ for life.
The confusion on bank sites arises because, until November 2010, the visa one got when applying successfully under SMC was called PR. (What is now PR was called PR with Returning Resident's Visa.) When INZ decided to change the terminology, they unfortunately used the same name again for something different. See here.
http://www.immigration.govt.nz/migra.../residence.htm A lot of institutions haven't even yet caught up, and changed their printed information or web pages.