20% of New Zealand will be invaded by wilding conifer forests within 20 years without rapid action. Wilding conifers currently cover more than 1.8 million ha of land, and are spreading at an estimated rate of 5% a year.
I was waiting for someone to mention this^^^. All we have to do is ignore the destruction of the original ecosystem by these conifers, and we have probably met or at least come close to the goal of 100mil trees per year.
Assuming they are spreading on the North Island central plateau, It will be a problem if they cover the Army bombing range. If a fire starts, the whole lot will have to be left to burn. As it would be too dangerous to fight the fire due to unexploded bombs.