https://www.cbp.gov/newsroom/stats/southwest-land-border-enc...
The chance of stopping it early increases exponentially, if there are fewer ways into your country from countries that are themselves exploding with the virus.
In Europe, all countries broke down eventually and all these cases are ultimately imported from crisis areas. Many were never caught.
Keeping borders open - in the EU sadly not something that could have been realistically changed, if ever - is the reason.
Look at the facts: NZ caught all cases early. Full stop. So did Germany - at the beginning. It worked in either case. But NZ also kept cases low by limiting travel. Germany did not do this - least of all because the EU literally sued Germany when it started (very recently) to lock borders.
Australia not only can limit travel easily, but is also huge and barely populated. When there was a significant outbreak, it was physically possible to cordon off the state in question.
In Europe, once the spread began, it was already present in every locality - since Europe is tiny and entire countries have higher population densities than Sidney or Melbourne! And, this is also important, there is significantly more travel of people and goods, and it is far less concentrated on a few areas. Many people in Europe go to work in a different country from where they live, every day. There are literally no borders where you could test people. It is being tried. It does very little. Look at the spread in areas bordering high incidence regions. The borders are as closed as EU laws allows. It simply does nothing.
This is it. If you closed borders, then you made it. If you did not, then nothing much helps except trying to reduce incidence rate such that hospitals do not collapse.
We all dreamed of containing non-regionalized outbreak clusters by contact tracing. In reality all health services, even the very best ones in the world, eventually got rolled. Some sooner, some later. Sometimes contact tracing worked up until the mutation showed up. But it never worked consistently, except with closed borders.
Which countries contained the spread while having a high density and without being an island? Vietnam. However, cross border traffic between Laos and China is either strictly controlled or not economically significant - in either case, Vietnam is also an example of a country that managed to lock down borders - even tourism - almost entirely.
Conclusion: Covid has to be geographically contained with maximum force. If that is not realistic, for example if your country has the density of a city, then you need to stop it from getting in in the first place.
And unsurprisingly, being an Island and not 100% dependent and interconnected with daily border-less traffic helps a lot. Being a huge country with less population density or other ways to cordon off locally also helps.
In Europe, several countries were doing fine. They had lockdowns, contained local spreads and did contact tracing. They did not do less than NZ or Australia - except they were still surrounded by dozens of other highly populated areas with entirely different policies, all with different timings. The result was the same everywhere - eventually.
So, theoretically the EU could have been saved, if border would have been closed off immediately after the first cases appeared in Italy. Despite the fact that people believed contact tracing was viable - an option that does not shatter the political and economic fabric of the region.
This post is already too long, but you can read up on the discussions at the time and the political, geographical, economical and demographic situation in the EU to convince yourself that the chance of early border closings was close to zero.
The average for island nations like New Zealand was 0.