IMO it's an 'artificial' league. IMO NZF should pull all stops out to get permission for a 2ND NZ A-league franchise then get rid of the ASB. The Chatham Cup used to be a good way of finding out the country's 'top' club. Chatham Cup winner and runner-up to the O-League. Sorted.
Not a bad solution. At the end of the day is trying to run an amateur (or at best - semi-pro) national league in a country the size of New Zealand just not viable?
Either that, or return to a club-based national league with strict financial controls and/or a player points system. You've still got the viability problem but financials controls might help with that.
Before you do anything you've really got to decide what the point of your league is though.
Is it about finding the best team in the country and having a decent run at the Club World Cup? And as Jerzy points out you don't necessarily need a national league to do that anyway.
Or is it a development league for youth, bridging the gap between regional football and one or more A-League franchises?