The weather in New Zealand is mostly consistent: assume it’s going to be foul and then every fine day is a bonus.
People from overseas are often surprised at how variable our maritime weather patterns are. It’s seldom stable for long, unlike continental weather patterns. That means it can go from being sunny and benign to the opposite in half an hour.
Then again, it seldom seems as bad once you are out in it.
The standard weather pattern for the South Island for much of the year is:
the arrival of a cold front with associated solid rain
slowly clearing for a few days of generally clear weather
new cold front, with associated solid rain
Heavy rain can occur at any time of the year but often it doesn’t last all day, showers or drizzle is the more usual weather phenomenon.
The best weather is usually associated with a large High parked out in the middle of the Tasman Sea. That can bring stable conditions for a week.
The most accurate weather forecast for the backcountry of New Zealand in my experience is from Norway, for some reason. Yr. no offers a ten day forecast for any named locality in New Zealand, including every DOC hut. If you want a weather forecast for some of New Zealand’s least visited huts, or anywhere else, well, this is the place. Like Sir Robert Hut. Just type in the desired locality into the search bar. There’s also an app for a cell phone.
Metservice gives a useful 7 day forecast aimed at the main NZ cities but also has some specialised forecasts for mountain regions. This is the national weather forecaster, used for radio forecasts, but suffers from one fairly serious issue: it just is hopelessly unreliable.
One of the better websites to predict weather is at the 7 day forecast page at MetVuw, metvuw.com/forecast/forecast.php?type=rain®ion=nzsi&noofdays=7. The whole site is somewhat clunky, ie, looks like 2002 revisited, but the information is fantastically useful, giving rain predictions in 6 hour increments for the next, err, 7 days. If you fiddle around there are enlargements for the South Island and you can find out when the next rain is due to come through. Oh, it’s here already.
There’s also this fantastically graphic website that shows what is happening with the wind at this exact moment, ie, right now, and that can give you an idea about what the weather is doing.
Wet weather gear
Due to wet weather often being associated with wind, it’s important to take rain gear that keeps you, mostly, dry. Lightweight raincoats can make walking in the rain a whole lot more miserable, and with wind chill can lead to hypothermia, ie, a dangerous condition of losing body heat that can cause disorientation and possibly death. Take decent wet weather gear.