seams to be a ton of poor info already.

first thing is nz houses are typically air leaky, they are not air tight.

use the bath fan etc, its good to get bad stuff out of the house, but thats not really a major cause. yes media makes a big song and dance about it but its not strictly true. i even dry clothes inside and the humidity level doesn't change.

all homes require ventilation. opening windows is the classic and you need a few open all the time. open windows is poor because the ventilation still depends on what the wind does. ventilation systems are a LOT better. but ventilation has limits depending on outside temps and humidity, and how well you heat your home.  even with a ventilation system your going to get some condensation on those god awful uninsulated window frames (imho the biggest con building industry pulled). but ventilation companies do a trick to make them condensation free, tho its not really good.

heating is part of the equation to. kiwis are really bad at heating whole homes. heat moves moisture and cold spots is where that moisture settles.

fortunately you need humidity above 80% for a long enough time for mold to grow (if it was 65% every home would be a mold fest). 

dehumidifiers, shouldn't really need one. ventilation should do it mostly. if for some reason you need it, refrigeration is fine. your house should never be that cold it performs badly. a desiccant dehumidifier is less efficient and costly to run due to the heat required for the regen. they are typically used when you want super low humidity levels, which you do not want in a home. you want 50% humidity, not 5% humidity. 

so put a ventilation system in, keep the windows closed, heat the house properly. put up with a bit of condensation on the lousy window frames nz builders wanted you to have.