I think this pretty much sums up why NZ is hesitant
"the Coalition was considering halving quarantine times to one week for countries with low rates of Covid-19 infection, including Singapore, Japan, Hong Kong and South Korea."
As soon as NZ and the Cooks "bubble" , the Cooks are open to any future bubbles NZ makes, and NZ is probably rightly concerned that the Cooks will go from "bubbling" to a country with 0 cases, to being tied into Australia that may have active cases, and shorter quarantine periods for other countries that may still have ( although at low levels) active virus....
This will be a big call for NZ too, are we prepared to give up our "elimination" status to join to OZ which may only be under a "supress" policy...