Food that is picked seasonally is more likely to stay on the tree until it is ripe. That way it reaches its full nutrient potential. And that's why just-picked produce tastes so good.
The longer the time between picking and eating, the fewer nutrients are left. Refrigeration and storage all increase the lifespan of produce, but as soon as it's harvested it begins losing its life force and nutritional value.
Eating seasonally means the food is more likely to be grown conventionally, within the natural life cycles. It's likely to have fewer pesticides, fertilisers and sprays to make it grow. All of these chemicals decrease the nutrient value of our food and we need more vitamins and minerals to deal with them.
Modern transport and food preserving techniques are new. Our bodies are made to eat different foods in different seasons. For instance, in summer many fruits are ripe. These are cleansing and cooling, and help us to deal with hotter temperatures. In winter many of the root vegetables are in season. These are warming and building, and help protect us from winter cold. Staying within natural life cycles makes sense to our body.
Seasonal foods are cheaper. They're grown locally and usually in plentiful supply. It's not often that the better option is less expensive!

The Tricks:

Buy fresh produce regularly, not just in your fortnightly shop.
Only choose produce that is grown in NZ, or as close to home as possible.
Choose fruits and veges that are in season. You'll know this by the greater availability of the product, its relative cost and its taste.
Buy organically if you can. Organic foods are always seasonal as they are spray-free and are not plied with chemicals to help them survive out of season. They are easiest to find at your local farmers market. The benefit is that they will have been picked the day before and so will be much fresher than what you typically get in the supermarket, and therefore packed full of taste and nutrients.

Gut Health

The Benefits of Eating Seasonally