Well, they did shared the hosting couple of times so it shouldn't be tough but ofcourse its all depends on the respective Gov. For the right reasons its their own choice if they are opening the borders for foreign players or not.
As of now, New Zealand is closed for international travelers. I don't think that they will make an exception, just for the world cup. If it was a handful of teams, then there was a chance. But the world cup involves a total of 16 teams and close to 300 players and other staff. So it is a bit complex. That said, Australia is not faring that bad. They are having only around 5-10 new cases per day.