its obviously very hard to quantify smuggling and the illegal keeping of reptiles. Not many snakes get found in NZ, and most of the ones that are have come in accidentally in cargo on ships or planes. There are a few cases of people being caught keeping them but only a few. No doubt there are people keeping them but it probably isn't a lot.
With lizards on the other hand, smuggling is rife both in and out of the country, because it has historically been easy to "launder" them as being bred in NZ. Loads of Australian species (birds as well) have been intercepted at the border over the decades: they are smuggled in, and then legally exported as "captive-bred in NZ" to foreign markets.
For local reptile keepers the availability of certain species has recently increased dramatically out of proportion to the numbers actually being bred, and colour morphs of things like bearded dragons and leopard geckoes are all over the place, even though these morphs weren't available before legal imports were banned.
Of course, you ask a reptile keeper here, and they would deny all knowledge of smuggling. The sad thing is so many of them are very anti-MaF and anti-DoC, and advocate (either indirectly or explicitly) smuggling and the illegal keeping of both native and exotic species. They feel it is their right to do so, and the government organisations are just trying to ruin their fun.
Some amusing examples of the stupidity of some of these people:
Baby green iguanas went up for sale on local auction site Trademe. The seller claimed they were bred from an old pair she had from the days before import bans but, oh no, that pair had recently been killed in a fire so she only had the babies left. All the babies were collected by MaF and DNA tested. They were from several different parents.....and some of them were Asian water dragons!
Another loser put green tree frogs (Litoria caeruleus) for sale on Trademe, claiming they had always been in the country. You know, which is why they had never been offered for sale before!! Interestingly enough, that guy has a brother in Australia who also keeps reptiles......
There was even one idiot a few years back who put a snake on Trademe for sale (I think it was a brown tree snake).
Basically there is a set list of reptiles allowed to be kept privately in NZ (see the sticky at the top of the NZ section of the forum). Anything else is illegal or probably illegal. Anything not on the list that is smuggled in needs to be kept quiet. Anything that is on the list can be smuggled in and distributed freely because there's no proof it has been smuggled.