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The MetService predicts severe northwest gales and 100mm-150mm of rain in South Westland.
Doc's Wanaka visitor information ranger Simon Mazzotti said if it rained as predicted, water levels could trigger a "code red" warning for the Young Valley, closing the area to the public.
"At this stage, we are putting out `high alert' warning signage in nearby visitor centres and at relevant track entrances.
"We are advising people to stay out of the Young Valley until further notice," he said.
The Young Valley landslip dam formed in September 2007, creating a lake which was 579.2m deep yesterday.
Doc and several other agencies implemented a code red warning system, in case the dam should burst and flood the valley. The warning threshold is 300mm of rain.
The MetService says winds are expected to become severe at times today in exposed inland parts of the lower South Island, with gusts up to 140kmh.
"Winds of this strength have the potential to damage trees and power lines as well as weaker structures and are likely to make driving hazardous, especially for tall-sided vehicles," the MetService said.
Fiordland could receive about 200mm-300mm of rain an hour, quickly raising the flows of rivers and streams. Headwaters of Otago and South Canterbury rivers and lakes faced a significant spillover risk.
Environment Southland's harbourmaster yesterday warned boat owners to check moorings were secure, particularly in Fiordland and on Lakes Te Anau and Manapouri.