Okay, you need to look at the INZ regulations, and old threads on the forum. INZ defines partnership as a live-in relationship, and you can't get partner-sponsored Residence until you've got evidence for 12 months living together. You'll have to wait a little longer to apply for Residence. Many people before you have discussed the kind of proof INZ will need. Here's your homework!
http://www.immigration.govt.nz/migra...artnership.htm
http://glossary.immigration.govt.nz/Partner.htm
https://www.google.co.uk/#q=site:enz...tnership+proof
'If there's a reason...' This sounds as if you suspect a sinister plot!
How's about innocent reasons like these? The mass of applications has to start out being separated SOMEHOW. You get applicants to divide themselves (rather than using one central address and having to share out in-house). It might actually be nicer for the applicants to deal with a place nearby, rather than having to send or phone to a remote location.
i've read somewhere before that applicants will be more likely to gain residency providing they run/own a successful business or earn a certain amount of income
Read the INZ operational manual, and you'll find that issues of having a business, or what your income is, only arise for different, e.g. entrepreneur and WTR, visas, not Family Stream. The different visa streams don't get 'mixed'. I know you're very keen for your partner's visa to go through easily - the way to make that happen is to concentrate on what the regulations tell you is relevant to his and your situation.