It all depends what you’re looking for as part of your wellness retreat. Given their location, the two islands are very different in terms of appearance and climate.
The North is much hotter, offering beautiful beaches, miles of forest and even volcanoes! Here’s the better island to choose if you’re into watersports and trekking in good weather. You’ll also find more of the North Island to be steeped in local Maori history.
By contrast, the South Island is much cooler, but has some of New Zealand’s finest lakes, mountains and glaciers. It’s also the place to go for those into alpine sports, having its own ski season from July to August. Visit both if you get the chance.