The issue is whether the captain was entitled to descend below the cloud layer on the basis that he was visual. In fact, he wasn't - but did he know that at the time? If he did, then it was a very bad case of pilot error, which bordered on outright recklessness.
If the relevant issue is the captain's knowledge, the lack of training provided by Air NZ has no relevance. If he already knew about the visual problem below the cloud, then what damage was done by Air NZ's not telling him? None.
Captain Collins clearly had a basic understanding of the problem. Note that we have snow in New Zealand, and mountains. For example, there is a Mount Hutt on the edge of the Canterbury Plains which, coincidently, is where the RNZAF Wigram base used to be and which is where the captain spent four years. When the cloud layer is lower than the mountain's summit, the mountain is not visible, which is an important piece of information for a trainee RNZAF pilot flying a Harvard over the Canterbury Plains.