Every home needs ventilation to bring in fresh air, to remove stale air and provide moisture control.
The traditional way to ventilate is by opening windows and doors. But this is not really convenient, and you cannot adjust neither the amount of fresh air you get (because it depends on weather conditions) nor can you control humidity this way.
Imagine buying a new car, where the manual reads: “To ventilate the inside of your car, open doors and windows frequently, and don't forget to close them again. To have some warmth, put on an extra sweater.” Certainly you wouldn’t accept that! So why should you agree to it when making a much bigger expenditure, namely buying a house? You can have even greater comfort and convenience in your home than in your car, why settle for less?
This is how ventilation with heat recovery can work (note: there are different systems. The following scheme is an example of a counter-current heat exchanger and an additional ground heat exchanger):

Winter scenario (click to open picture):

(1) Stale air is exhausted in parts of the house, where moisture and smell is produced predominantly (like kitchens and bathrooms)
(2) Fresh air is transported to the living- and bedrooms, passing through an air-to-air heat exchanger (3), which is warmed through the exhausted air. Both airways are strictly separated by a thin membrane
The fresh air is preheated by a brine-to air ground heat exchanger (4). The brine is circulating through the ground and thereby warmed. In NZ weather conditions the difference between ground temperature and outside air temperature on a winter day is not as considerable as in mid Europe , so the effect wouldn’t be worth installing a ground heat exchanger in most parts of the country. However there are significant benefits in summer.

Summer scenario (click to open picture):

(1) Stale air is exhausted in parts of the house, where moisture and smell is produced predominantly (like kitchens and bathrooms)
(2) Fresh air is transported to the living- and bedrooms. The air-to-air heat exchanger (3) is bypassed
The fresh air is pre-cooled by a brine-to air ground heat exchanger (4). The brine is circulating through the ground and thereby cooled. By passing the cool brine the incoming air will give up a proportion of their moisture load through condensation. The condensate is collected and led into the sewage system. The air is thereby dried and slightly cooled.
When it is neither particularly hot nor cool, you might as well switch off your ventilation system and open some windows. If you suffer from an allergy caused by air bound allergens (there are filters on both sides of the airway) or have particularly noisy neighbours you’d like however run the ventilation system year round.

Installation tip:
never use flexile ducts for ventilation purposes. They increase the amount of energy needed to transport air within, because their accordion like surface exalts resistance. In addition they are prone to contamination and almost impossible to clean either. Choose tight materials with a smooth surface instead. For examples see www.lindab.com