I agree with the others, from what we know of the story, it looks like the hikers more or less did everything right. Stayed put, rationed their food, and opted to hike out under their own power once the conditions settled down. Presumably they could have given a more detailed itinerary on when they planned to return from their trip, but otherwise, it seems like they kept a cool head about their situation.
I spent about 9 months in NZ spread out over two back to back NZ winters a few years back and did a lot of winter tramping, primarily on the South Island. Like one of the other posters said, a weather forecast going out more than 3 days or so, especially on the south island in the winter, is worthless. I recall getting caught by surprise by the weather on a few occasions either by a storm moving in early or hitting with a lot more punch than had been expected. I spent a few unplanned nights out waiting for storms to settle down or river levels to subside.
Sitting in the various huts, reading the entries in the hut registers, there were numerous tales of unprepared, inexperienced, overly ambitious or just plain old unlucky trampers getting caught out in bad weather and/or making bad decisions that resulted in them needing to be rescued or at a minimum being supremely humbled by the forces of nature.