South Pacific experts - any advice?

We are embarking ona RTW trip and want to include sOme parts of the South Pacific. At the moment we are thinking cook islands and maybe one other location. We did not plan to book his into a RTW flight ticket. Anyone know if this can be booked through an agent in NZ and any idea of costs for say a 1 month jaunt into these places?
We will likely be staying in backpacker places with possibly the occasional splurge (I know splurges are of the most expensive kind there)
Finally when is the best time of year to visit?

Thanks

At the moment we are thinking cook islands and maybe one other location.

As it seems that you want to depart from NZ, if you don't want to backtrack after visiting the Cook Islands, the only possible other location (with direct flights to Rarotonga) would be Papeete in Tahiti (French Polynesia).

I would generally advise people wanting to visit 2 countries to combine one Polynesian country and one Melanesian (Vanuatu, Solomon Islands or New Caledonia). Fiji (Melanesian people with a Polynesian culture) can provide a cross-roads, as many pacific countries have flights to and from Fiji.