I haven’t studied this issue (hence my question) but that never stopped an opinion so here is my 2 cents:
I think whoever posted it would be the toilet paper battles times 10,000,000 was not too far off the mark. But it would be an evolving situation. People would know what was coming from the news, and would then become very afraid when communications ceased. I think there would be panic and riots as supermarkets were raided. But this wouldn’t last. After a few days, the panic would subside and people would start strategising for survival. I think law and order on a local scale would emerge and most communities would come together against gangs and anti-social behaviour. But there might be armed conflict along the way.
In the longer term, there might be tribalism as communities struggle for resources. Yes, New Zealand is blessed with land for growing crops, but clean water is in short supply and 19th century agricultural techniques, which may have worked for a population of 100,000, will be challenged to feed five million. There will be much strife and hardship, especially in urban centres.
The lack of modern communications would probably result in the collapse of national government and the emergence of localised power bases. Limited trade would come about.
There might be invasions of overseas refugees. Some would be repelled, some might overwhelm local defences. As things became more settled, it would be harder to get in.
We would survive, at least in the short term, but we might not enjoy it much.