raytaylor:

 

Even so, its still best for troubleshooting purposes (the isp can see things in the chorus troubleshooting portal like power and lan port status) if the customer calling them can physically access the ONT. 

 

So all you need to do is just get an opticat cable from the existing ONT or ETP and route it to the location you would like the new ONT installed. Then the chorus tech will come out, install the ONT and splice the cable at no charge. 
If you dont have the skills to run the cable yourself, you can hire an electrician or low voltage tech to do it. 

 

When I had internet issues the One NZ tech told me a couple times to restart both HFC modem and Router, so thought this was normal for an ONT as well. From the comments am guessing that won’t happen too often when we change from HFC to Fibre.

So it sounds as if the easiest option is to go with the main ONT's port 2 cabled to the downstairs attached to a second ONT for there. Then they can get whichever ISP suits them.

This brings up other questions. When the downstairs with a different ISP to mine have a fault, will this impact my ONT upstairs since their optical cable is coming from my ONT or would most issues be fixed via his ONT/router. 

Will downstairs turning off his ONT affect my internet? I suspect that turning off my ONT upstairs will cause downstairs an issue and not the other way around. So many variables to get my head around.

Since I don’t have much faith in my older sparky's ability with tech for any cable reconfiguring, when the time comes maybe I can DM the knowledgeable @nztim again?

Apologies to OP and don’t mean to hijack his post but sounds like we may have similar situations and advice will benefit us both.