Historically New Zealand has welcomed immigration. These days 20-25% of our workforce was not born in New Zealand (similar levels to Australia and Canada). Our natural rate of population increase is around 0.5% pa and that is falling away as the population ages. Net migration adds 1-1.5% pa to population growth.
Net immigration puts a small upward bias to economic growth which is good for keeping confidence and encourages investors to take the risks necessary to underpin growth in per capita incomes.
Migrants tend to be younger than the average age of the population and so add to our productive capacity, which of course is what funds social services like pensions and public healthcare. And of course we will need migrants to provide services to an ageing population also.
So there are plenty of arguments for it, but these need to be tempered by our ability to host more people, and the quality of our migrants in terms of their ability to boost the per capita incomes of the existing population. It is on these issues that immigration policy is often criticised.