It's not just NZ. Australian states have carried out similar decisive actions.
An example. We had one person in Brisbane (capital of Queensland) who was infected with the UK variant whilst working in one of the isolation hotels (used to isolate people returning to Australia for 14 days).
The infected person reported that they had travelled extensively through the greater Brisbane area during the previous days. Given the unknown factors around the increased ability to transmit of the UK variant, the Queensland government called a 3 day lockdown on Brisbane. No travel unless absolutely necessary (food, medical care, critical job) and if you were outside your house for any reason masks were required.
The three days were to allow the health department to trace all the contacts of the infected person and get them tested. General public were also told to get tested if they had any symptoms and testing facilities were reactivated across the area.
Three days later, no new infections and the restrictions, only retaining the mask mandate in confined areas for another three weeks. We are now about three weeks later and no new community cases.
So was the three day lockdown excessive. I don't believe so. Would the outcome have been any different if we didn't have the three day lockdown? As it turns out, probably not, but it has to be noted that the purpose of the lockdown was to force a stop in possible transmission and give the health department a chance to find out how many people had been infected. Turns out that the infection rate was very low (some immediate family).
If we'd found from the tracing that there was active community transmission there would have been an extension to the lockdown until the rate reduced.
But here we are, essentially living lives unrestricted our freedoms intact until the next time this thing escapes into the community. Then I trust that the government will take prompt action and the public will groan but comply and we will be inconvenienced for a short period before we again return to normal.
Australia has learned a lot about what works and are acting on it. I don't know what lessons can be transferred to the tragic situation in the US apart from showing what would have been possible if your government had taken this seriously from the start.
I feel very sad for your country and wonder how the greatest democracy on the planet has allowed individual freedom (if you want to call it that) to be considered so important that it has been allowed to compromise the freedom of all.