Long story short..... There will not be enough Eagle pilots to cover the requirement for pilots at NSN/Cook. Hence the reason to externally hire.
Especially if you take into account that Eagle require people to stay behind for the operation to continue. These people will have a Jet/Cook/Nelson seniority number to protect their position, but remain at Eagle on secondment.
I believe at all regional yes letters have been offered at Eagle start ( starting very shortly ) and that Eagle will also look to take 10 graduates from the AirNZ Airline Integration Course.