While the Caketin is a new stadium, it is not rectangular, and it was designed to accomodate cricket (like the stadia in the other main cities like Eden Park and Christchurch). Having watched League and Union there, there are parts of the stadium are distant from the pitch. Would the second team need to be based in Wellington or Christchurch? There are several potentially better options for another NRL team:
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North Harbour (Albany) - ignored for a Super Rugby franchise, yet it has a good rectangular stadium (unlike most major NZ grounds), and Auckland as a whole has a large playing base and market in both codes. 2/3 of the RL player base (and probably fan base) comes from the greater Auckland area anyway. That could build a strong local rivalry for RL. IIRC previous private bidders for the Warriors came from Auckland.
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The Waikato - its only professional sporting franchise is the Chiefs, who play in Super 14. The region has produced a number of NRL players like Hohaia and Koopu, and Hamilton has a good dedicated Rugby stadium with a rectangular pitch.
* Other regions like
Taranaki (New Plymouth) and
Hawke's Bay also bid for a Super 15 franchise in Rugby, but the NZRU rejected them as they only want to support 5 franchises.
Nevertheless, North Harbour and Waikato could be realistic contenders for a 2nd franchise. Splitting Auckland between the Warriors and a Northern team would still allow them to draw upon bigger markets than Christchurch or Wellington, and Waikato (and its surrounding regions) is not much smaller than Wellington:
Region - Population
Auckland - 1,414,800
Canterbury - 552,800
Wellington - 473,700
Waikato - 402,200