One of my most visited sites for weather is www.metvuw.com The 7 day forecast for the South Island is remarkably accurate at ground level for precipitation and more importantly, wind direction.
Another useful site as it gives accurate wind speed at different levels is www.metservice.co.nz The best categories are the national park ones, and Wanaka backcountry rivers tend to be a mix of the Southern Lakes and Mount Aspiring areas.