We could never completely close down our borders. International law prevents it. For a start what would happen to diplomatic travel? That is constant and necessary to keep New Zealand operating in the world.
What about the export and import of goods? The last time I looked aeroplanes and ships need people to operate them and load and unload them.
Unfortunately the Government has sold us (well not all of us) a line that was never ever sustainable. A bit like RITA really. New Zealand cannot function as a completely isolated island nation waiting for some miracle Covid-19 cure. The first Level-3 and Level-4 lockdowns have to date cost us over $100 billion! To put $1 billion in perspective. One billion horses nose to tail would circle the earth 59 times!
We are borrowing $77 million dollars an hour! $2 billion a day! As a country our only way of paying that back is to trade be it produce based or human based (e.g. tourism and education). At $2 billion dollars a day you can't keep our borders closed for very long.
No our only option now is what we should have done in the first place. Take a non-isolationist approach and MANAGED the pandemic. Not put our heads in the sand. The 80,000 deaths prediction was a lie and has proven to be one. In Sweden the same model said 96,000 (even though they have twice the number of people as us!) and they have had under 6,000 deaths. If they had managed aged care facilities better then that toll could have been conceivably half that or 250 fatalities/1 million people. That would equate to 1,250 deaths in New Zealand NOT 80,000. That would have taken our yearly death toll from 45,000 to 46,250!