The West Coast is wet year-round — Westland receives up to 5,000mm of rain annually, and the mountains above the glaciers considerably more than that. This is not a destination where you can time a trip to avoid the weather; you accept it as part of the experience. That said, summer (December–February) is warmest, and spring and autumn can produce clear windows that feel like a reward after days of grey.
For glacier visits, go early in the day before cloud builds on the mountains and obscures the ice above. Lake Matheson reflections require a calm, clear dawn. For wildlife, winter (May–July) is when Fiordland crested penguins moult at Monro Beach, and kiwi spotting at Okarito runs year-round but night tours are easiest to find in summer. Snow on the Haast Pass is possible in winter but rarely closes the road for long.