The Southern Ocean and Tasman Sea have seen a series of strong storms over the past week, but the strongest storm yet is likely to occur on Sunday for NZ's South Island. A deep low is expected to form just south of Tasmania on Sunday morning and is forecast to move east along the southern Tasman during the day. This true "Winter Storm" will buffet the south coast of the South Island with gale to storm force winds overnight Sunday into Monday, until the low slowly drifts to the southeast. The winds and the amount of cold air set to affect the South Island, will likely be some of the strongest and coldest of the year so far. Heavy snowfalls will help the struggling snowfields, as well as snow being seen down to some of the lowest levels of the year to date. Sunday night will see average winds in the 60-80 km/h range for most areas of the South Island. Gusts are likely to exceed the 100 km/h for areas of Southland and Invercargill with exposed coasts and elevated areas seeing isolated gusts to 115km/h. Lastly, the large waves will continue to affect western coasts of both islands with dangerous waves on Sunday reaching heights seen only once a year