Back when I submitted my application in March 2019, I printed the INZ 1104 and INZ 1235 forms double-sided, and the majority of supporting documentation single-sided (I think that some supporting documentation was double-sided, but I can't remember for sure). INZ didn't have a problem with this at all.
This is the section on the INZ 1105 guide (Application for Residence Guide - Skilled Migrant Category) that describes the documents you must send (and this can be considered as authoritative a source as it gets):
In order to make a decision on your application, we need to see documents to support the statements you have made in your Expression of Interest.
All documents you supply with your application should be either originals or certified copies of originals. If you send certified copies, we may ask for originals at a later date. Your Invitation to Apply for residence may include instructions about whether you should provide original documents or certified copies.
‘Certified copies’ are photocopies that have been stamped or endorsed by a person who confirms the copy is a true copy of the original. The person who certifies the copy must be authorised to do so by law, in your home country or in New Zealand (eg a Justice of the Peace, notary public, lawyer or court official).
Before we can make a decision on your application, we may need to ask you for further information or evidence not listed in your Invitation to Apply. We may also need to verify the information in these additional documents.
There is no information in the INZ 1105 guide, or in the operations manual that I could find (I looked at SM3.10 and R2.55 specifically) that specifically mentions whether INZ wants supporting documentation single-sided or double-sided. Given this, my own experience above and the fact that any printed application form that one can pick up at a Citizen's Advice Bureau is double-sided (implying that INZ are quite happy to accept double-sided paper application forms), one can reasonably infer that it makes no difference whatsoever. The likely worst case scenario is that it will just take the clerks longer to process the lodgement of your application.