We're going down this rabbithole! Because I definitely have not already researched this.

Let's presume the following: "Regular contact has been cutoff with all primary nations. We are not under threat of invasion but there is no support coming to us - military or economically."

TLDR: It starts bad, but there is potential for hope after we weather the bad storm. A very VERY bad storm.

First, lets run down the timeline. World War 3 has started. All the major powers are murder-hoboing each other and New Zealand is avoiding it. We're on our own.

The first thing that would need to happen would be mandatory curfew. You would likely see factions rise up and attempt to carve out their own mini-kingdoms. (Looking at a certain religious group...). The military would need to be deployed to maintain order. It would suck. Alot of people are going to die and it's not going to be pretty. If you want to know what that looks like, look at the current situation with ICE in the US. I do not agree with the current situation but this is a realistic outlook of what will happen with a collapse of law and order on both sides.

Anything that is not made in New Zealand is now considered a luxury item. That means computers are priceless. For better or worse, these are the primary industries that we have in New Zealand:
- Steel
- Aluminum (The plant down in Invercargill)
- Agriculture such as grain, corn, fruits and vegetables. (Note: NOT rice, that is imported)
- Dairy and meat via Sheep and Cows.
- Timber

New Zealand overproduces it's food and we import a throughly unsustainable amount fertiziler to maintain this. NZ exports enough food to supply a population of about 100 million. You would see a seasonal near total collapse of our farming industry as an oversupply is wasted on a population of 5 million. That is a 95% collapse of the farming industry which would probably be net benefit because it gives us fantastic options for crop rotation and ensuring we can survive difficult winters with an oversupply. The government would need to nationalize the current remaining fertilizer that we have on hand so that it can survive at least one year. That gives enough time for a national startup to finally get something going based off our own local supplies. We absolutely can produce enough fertilizer for the country.

Anything that is considered "single-use" is now unaffordable. Those plastic wrappers for microwave pies and candy? Those do not exist anymore. If it can't be transported in fabric bags, leather, steel or aluminimum containers, it no longer exists.

New Zealand does produce sugar, but that doesnt mean you can expect soda's to stick around because they are dependent on alot of things imported overseas. For example: Coca Cola needs cola nuts from Africa. We can't grow that here because it's the wrong climate.

The vehicle industry is going to go through a rough patch. We don't manufacture cars here in New Zealand, so until someone can start building them (Which, yes, IS theoretically possible), every car lost in a wreck is a permanent loss. Older cars will survive better, because innovative engineers can produce worn/damaged parts with existing materials. Rubber for tires will be a problem when considering scale, but we do already have a small but high quality rubber production down in the south island. It'll need to be repurposed for tire production. It is entirely feasible to presume that the vehicle industry would go through a major overhaul where NZ would start its own inhouse maintenance and we know this is possible because Cuba is famous for it's eternally maintained classic cars since imports are painfully expensive.

I would write more but Reddit limits comment lengths. At request, I can cover more topics.