The infiltration rate of a traditionally built, draughty home is subject weather conditions. In cold and windy weather there is plenty of it. On a calm day in spring on the other hand, infiltration and ventilation through windows may be next to zero. You should not rely on the weather to get ventilation right. The first step to achieve this is getting your house airtight. This is done by installing an airtight barrier on the inside of the insulation layer.
Most membranes you see these days, though, are installed on the outside of walls. You might need a barrier there too, as a second line of defence against wind and driving rain.
A membrane that provides airtightness is however needed on the inside of the insulation layer, to prevent the moisture content of the warmer indoor air to condensate on a colder layer somewhere exterior of the insulation layer.

Airtightness reduces energy losses to some extent, and makes your home more comfortable to live in - but more importantly it prevents moisture from building up in construction cavities. Proper ventilation of your home is of course needed, you just should never try to reach it through cracks and joints, though. Ventilation through uncontrollable gaps is always insufficient; in addition, there is a good chance to harm your construction, literally on the way (of the moisture laden air ways). Therefore, think of proper ways to ventilate your home, preferably through an automated ventilation system with very good heat recovery (80+%, verified by an independent body – they all say they are excellent, but some are not).

Whenever you have a fire burning inside your house other than for mere cooking purposes you need to provide external ventilation, because you do not get a fire maintained safely without dedicated ventilation. You can use an airtight and independently ventilated wood burner. However, you will use this fireplace only in inland areas or if you either like it really cosy or are very fond of gazing at the flames. You will not need it to keep a Passive House warm in most NZ locations.

Airtightness can be measured. A blower-door is the preferred tool to do that. See http://www.blowerdoor.de/airtightness/BlowerDoor.htm for more info, on how it works.
I did numerous blower-door tests, and find it a very easyly applied method to detect air leaks, before they can do any harm.
An air exchange rate below 0.6 per hour (under 50 Pa pressure difference according to EN 13829) is highly recommended if you want to operate a ventilation system with heat recovery successfully. Otherwise, a large amount of air enters the building without passing through the heat exchanger, and interior warmth is discharged unused.