It buys time to prepare, and for vaccines and other treatments.
By closing their borders it gives them time to build up their healthcare infrastructure, wait for vaccines and other experimental treatments to be vetted so by the time their borders open up again it won't devastate the country like it has everywhere else. These are all luxuries every nation wishes it could have taken advantage of. Criticizing New Zealand's response right now is extremely premature.
It may surprise some people but the number of Australians that visit countries overseas is roughly equal to tourists visiting Australia. As wealthy diversified nations tourism for Australia and New Zealand is a net wash in terms of foreign cash flows. Citizens spend as much overseas as tourists coming to Australia. The issue isn't foreign currency gains/losses but a more simple case of the economy and jobs are setup to service a certain number of tourists who are no longer there.
Both nations leaders have met and discussed encouraging bilateral tourism. Honeymoons, family overseas trips, etc. will be bilateral and internal for the time being. If the previously external tourism can be directed internally it'll actually more or less be a wash on topline numbers.