A front moves onto the lower South Island tonight, then crosses the remainder of southern and central New Zealand during Thursday. A strong, moist northwest flow ahead of this front is forecast to bring a period of heavy rain to the west of the South Island. About the Westland ranges south of Otira, 150 to 180mm of rain is forecast to fall from tonight until mid-morning Thursday. This amount of rain is likely to cause rivers and streams to rise rapidly, and may lead to surface flooding and slips.