I have a slightly different theory/perspective... I don't know so much about the rugby side.
But because Rugby Union is so dominate in NZ as apposed to Qld and NSW, and League is a very different beast to Union. In NZ you do not get most talent coming from grass roots League where you learn early about the finer aspects/intuition/skillsets/feel of our Great Game and have to dip into the Rugby Union pool of players.
Look at some players who were born in Aussie or went over as kids to learn their trade, and have played in the NRL like (born in Aus) the Cayless boys, Kalyn Ponga, Josh Hoffman, Jason Nightingale, Frank Pritchard, Gerard
Beale, or went to Aussie as youngsters, Jason Taumalolo, Kieran Foran, Karmichael Hunt, Alex Glenn, Joe Ofahengau and probably a lot more to name...