It all depends on who you ship with. From the NZ end, there is no requirement if you want to load up the van on or not. The reason why shipping companies don't want customers loading up belongings into their vehicle is becaues of the hassles. Hassles like quarantine and additional fumigatiion done to the container caused by having such goods loaded. Hassles like items going missing because the person moving the vehicle out may see something they like.
If the shipping company doesn't want anything in the van, they should at least give you the benefit to partition the container. For eg, in a 20 ft' container your van may be 15 feet long. The 5 feet of extra space you frame out, you can pack what ever you want. Make shelvings etc. but the wood must be approved for use because if it isn't, then MAF in NZ will quarantine the whole container as they would expect fumigation of the whole container.