The heavy rain and severe gales hitting parts of Canterbury, Otago, Southland and the West Coast are expected to continue into tomorrow.
A MetService spokesman said a front, preceded by strong and moist northerly winds, was expected to move on to the South Island from the west today, then move slowly northeastward during tomorrow.
An Orange heavy rain warning has been issued for the headwaters of the Canterbury lakes and rivers about and south of Arthur's Pass until 9am on Thursday.
An Orange strong wind warning will remain in place for the Canterbury High Country until 6pm today.
"The front is forecast to deliver heavy rain to the west of the South Island, including the headwaters of the Otago and Canterbury lakes and rivers.
"Severe gale north to northwesterlies are forecast about the Canterbury High Country, Fiordland, and western parts of Otago and Southland.
"Warnings and watches for heavy rain and severe gales are in force for these areas."
"Expect 400 to 550 mm of rain about the main divide, with more than 120 mm of rain within 30 km east of the divide. Peak rates of 20 to 35 mm/h about the divide during Wednesday, and 15 to 25 mm/h on Thursday morning.
"Heavy rain may cause streams and rivers to rise rapidly. Surface flooding and slips are also possible and driving conditions may be hazardous."
Strong winds gusting up to 140km/h are also expected for the Canterbury High Country, he said.
"Strong wind gusts could damage trees, powerlines and unsecured structures. Driving may be hazardous, especially for high-sided vehicles and motorcycles."
The spokesman said the amount of rain forecast for central parts of Westland, in particular, was "significant".
"Dangerous river conditions and flooding are possible, with the potential for slips and floodwaters to make some roads impassable.
"An Orange warning for heavy rain is currently in place for Westland, but may be upgraded to a Red warning closer to the time.
He said Westland and the headwaters of the Canterbury lakes and rivers south of Arthurs Pass could expect up to 550mm of rain between this morning and 9am tomorrow, with peak rates of up to 35mm per hour or more today.
The headwaters of the Otago lakes and rivers could expect up to 150mm of rain about the divide, and up to 100mm within 25km east of the divide.
Peak rates of up to 30mm per hour were expected before dawn today and heavy rain may return later today.
He said it was possible the weather warnings would still be in place on Friday for Westland, and extended to all of Fiordland and Stewart Island.
"A cooler southwest change is expected on Friday.
"This may bring some snow to the higher roads of the southern South Island, but it is unlikely that snow warnings will be issued.
"However, [it is possible] that strong west to southwest winds associated with the change will require a warning for Stewart Island and parts of coastal Southland and Clutha on Friday and early Saturday."
A large high was expected to move on to the country during Saturday and Sunday, bringing easing conditions, he said.
