Another idea for what it is worth but you can have both One NZ and Spark networks active on the same phone or have a network each, then you at least get what coverage there is.
I have recently went to having both networks on my iPhone. Company pays for main one, I pay minimum to have the backup. I had put it off for ages but went in, took 5 mins and so far no issues. Of course I now have another number so updated contact details and sent to family only.
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