Damaging hurricane-force winds over 150km/h, waves as much as 13 metres tall and a metre of snow for the Southern Alps are forecast with two Southern Ocean storms set to slam into New Zealand.
Two significant lows - one today and the other around Tuesday/Wednesday - are due to blast the country, with urgent warnings for anyone caught outdoors.
"This would be a significant and serious weather event for New Zealand in mid-winter let alone smack-bang in the middle of summer," WeatherWatch says.
The first low on Sunday is forecast to bring thunderstorms for many areas today, especially southern and western regions.
But the storm next week has the potential to be much more serious, especially for those outdoors, tramping or camping or on boats.
"The Tuesday/Wednesday event looks to be most intense with the centre of this storm potentially crossing Southland and Otago," WeatherWatch warns.
"As we see this storm moving through it will first push damaging nor'west gales up the nation then be followed by a wintry southerly change, mostly impacting the southern half of the South Island whilst bringing heavy rain to the West Coast."
The main risks
Wind
Peak winds will be on Tuesday with severe gales likely in both islands.
"The North Island is mainly impacted by gales on Tuesday/Wednesday in southern and eastern areas in particular," WeatherWatch says.
Wellington and Wairarapa will be especially windy on Tuesday, averaging gale-force and gusting over 120km/h in the city centre. Gusts on the Wellington hilltops may be over 150km/h.
"Boats and chartered services around the South Island, Cook Strait and western North Island may be cancelled or delayed due to hurricane-force winds at sea and dangerous swells," WeatherWatch warns.
"Some flights over Tuesday and Wednesday in both islands may be impacted by gales."
Seas
Major waves are coming for the southwestern corner of the South Island - around 12 to 13 metres tall.
"Dangerous rips and waves will affect much of western New Zealand this coming week and southern regions too," WeatherWatch warns.
"Boats and chartered services around the South Island, Cook Strait and western North Island may be cancelled or delayed due to hurricane-force winds at sea and dangerous swells."
Rain
WeatherWatch warns of heavy rain on the West Coast with accumulations in the mountains potentially reaching 200 to 250mm over the next five days.
"This is very heavy and could lead to slips, flooding and even road closures. Swimmers should avoid streams and rivers while in flood and until waters clear in the days following," WeatherWatch says.
Temperatures
There'll be a huge drop in the south, with some places in Otago, Southland and Fiordland dropping to single-digit highs on Wednesday.
"In the South Island the days ahead will be much colder than usual with some places dropping over 10C below normal on Wednesday," WeatherWatch says.
Snow
WeatherWatch says there could be a metre of snow on the tops of the Southern Alps.
"Heavy snow may impact walking tracks at higher elevations in the South Island (mainly higher altitude mountains above the tree line)," it says.
"We have snow flurries or sleet possible in Arthur's Pass this week too, likely mixed in with some rain."
