Looking for cheap one week SP beach break departing from Christchurch, NZ

I live in NZ. I'm craving some swimming/beach time, but I already took my main holiday for the year. I'm trying to figure out if I can go somewhere in the pacific for a cheap, one week break over our summer (I can go early December if I need to travel before the prices jump up for Xmas/school holidays). What has put me off going to the SP previously has been the (hopefully wrong?) idea that I'm going to need to take an expensive internal flight once I get there to get to the places I'd really like to go (e.g., Aitutaki - I really wouldn't want to go to Raro and not go to Aitutaki which appeals far more than Raro itself). Does anyone have suggestions of where I could get to cheaply from Chch NZ that has really great swimming -

• the criteria for this for me are: ultra warm, calm, clear water with no significant waves and no sea lice

(Perhentian Besar is my current favorite island destination if that gives people an idea of my prefs).

Any suggestions of somewhere that would meet the criteria without needing an expensive internal flight once I get there?

Fiji and Tonga are the cheapest countries in the region, and are probably the cheapest to fly to from NZ, too. Both have a decent domestic shipping network (more extensive and comfortable in Fiji) so you could visit different islands in them without having to fly. And of course both have many islands with warm, clear water.