Up to 200mm, even 300mm, of rain is expected in the ranges of Westland and Fiordland between Otira and Milford Sound overnight.
Heavy rain is also expected to spill over into the Canterbury headwaters, the ranges of Nelson and Mt Taranaki. Rivers and streams are expected to rise quickly and surface flooding and slips are possible, MetService warned.
As well as the heavy rain, the severe weather warning also covers a threat of severe northerly gales in Wellington this afternoon.
It also warns of severe northwest gales about exposed inland parts of North Canterbury, Marlborough and Wairarapa from this evening to early tomorrow morning.
"Gusts may reach 120km/h in these areas during this time, especially about exposed hilltops,'' MetService said.
"Winds of this strength could damage trees and powerlines and make driving difficult.''
In the main centres:
- Auckland tomorrow will have sunny spells, a westerly breeze with possible afternoon showers, and a high of 22 degrees. Rain arrives Wednesday night.
- Hamilton is looking at 21 degrees tomorrow, cloudy periods, a northwest breeze and a few showers. Wednesday looks much the same, a high of 19 degrees.
- Wellington can expect sunny spells, a high of 18 degrees and easing northerly gales tomorrow. But on Wednesday, showers turn to rain.
- Christchurch starts tomorrow with 19 degrees thanks to gusty nor'westers, easing throughout the day. A few showers on Wednesday.
- Dunedin has an umbrella of variable high cloud tomorrow, gusty norwest winds, and a high of 16 degrees. Showers start Wednesday and will hang around the rest of the week.
