Here
https://www.immigration.govt.nz/new-...-resident-visa is the INZ web page with links to all the information for the partner-sponsored residence visa. The WHV will be no bar to applying whenever you want, once you have solid evidence of at least 12 months' living together. See old threads
https://www.google.co.uk/#q=site:enz...tnership+proof for discussion about evidence, and suggestions that may give you more ideas you can use.
And here
https://www.immigration.govt.nz/new-...anent-resident is the page about PR. Be aware, if she's going to use the 'time in NZ' criterion as her proof of commitment to New Zealand (which most people do), INZ count BACKWARDS from the lodgement of the application, counting DAYS exactly. So she would have to have spent at least 184 days in NZ in the 12 months leading up to the date of lodgement, and again, 184 days in NZ in the 12 months before that. You're correct that a NZ PR holder can spend any amount of time outside the country and return with full rights to live and work again whenever they wish.