And THERE (I think) is the logic behind the NRL pumping for NZ 2 as team 18 instead of Perth.
If it's the other way round (Perth as 18, NZ 2 as 19), TV gets an extra game AND a late night time slot all at once... so there's a risk the NRL could get shortchanged (like a kid born on December 22 that gets a combination Christmas and Birthday present).
By drip-feeding "features of value" to broadcasters, they get a decent slice for NZ 2 (team 18, extra game), and again for Perth (team 19, late night games!)
Contrast that with Perth as 18, then NZ 2 as 19 - Fox/Nine may say "OK, more money for an extra game per weekend & a new timeslot" when Perth is added as 18, but when the NRL tries adding NZ 2 as team 19, Aussie broadcasters will say "Well, what's in it for us? There's already a NZ club, and going from 18 to 19 doesn't add a game each weekend..." - a really tough sell.