NZ was found to be COVERED in mould- the interior was wrecked so make sure it's bone dry before you ship it.
The reason why the you got mould all over the car is because the container had sweated. Other than shipping the vehicle in a temperature controlled container (which very few people would pay the extra $ for), there's is not much you can do to control container sweating. It could be that from the UK, the shipping companies go through a long journey of ports all over the world before arriving to NZ. UK is very far away from NZ and the container may change carriers often. Travels through high humid places in Asia pose a problem with mold growth.
I had a car shipped from Vancouver, Canada to NZ many years ago. I specifically picked the shipping route from the carrier that was most direct to NZ with fewest stops (Vancouver -> Long Island California -> Lyttleton Port Chch NZ). I paid a premium but when the container arrived in NZ, the MAF officer and I witnessed not 1 singled drop of water was on the body of the car. No water marks means no sweating. After presenting a receipt from a Vancouver detailer outlining the cleaning process done on the car, the MAF officer had a quick look under the car, under the bonnet, and under the wheel wells - he was satisfied.
You can inform MAF that you've cleaned the vehicle yourself but that's not their concern. They need to be satisified that the vehicle was truly cleaned and this usually means that the underside is never cleaned properly by the vehicle owner. Choice of vehicle plays a big role here. Are you bringing in a show car or a daily user car? MAF inspectors usually know once the door is open. "Yes this looks like an expensive classic car in Concours' condition". Meaning, such vehicles don't go to the car wash, rather, they go to a "detailer" where the underside of the car is hand washed and wiped. What i'm getting at is you need to demonstrate to MAF that the vehicle is presentable. Proper detail cleaning and methyl bromide treatement prior to leaving the country.