A giant storm growing in the Southern Ocean will lead to "gigantic seas and hurricane-force winds" off New Zealand.
The huge area of low pressure will stretch from Antarctica to New Zealand next week, creating a large storm which will span thousands of kilometres in diameter.
"This is definitely one of the larger storms we've seen this year," WeatherWatch says.
It warns this monstrous weather system will bring heavy rain and gales as the edge of it brushes New Zealand.
And it will mean a "terrifying ordeal" for any ships caught in its path as it churns up the Southern Ocean.
Which regions will be worst affected
The West Coast is back in the firing line, with WeatherWatch warning rainfall totals will be "excessive".
"Most impacted by rain will be the West Coast with significant totals there which could lead to slips, flooding and road closures," the weather forecaster says.
"Over the next seven days nearly half-a-metre of rain is possible. Around Franz Josef the totals could be 400 to 500mm with 250mm as far north as Westport."
Strong gales are also forecast through the mountains, to the east and about Cook Strait.
"On Monday night and across Tuesday a surge of gale-force northwest winds moves up the South Island, Cook Strait and lower North Island. This typically mostly affects the eastern side of the South Island and Wellington/Southern Wairarapa. Wind warnings may be possible for a time," WeatherWatch says.
"Over Wednesday/Thursday gusty northwest winds blow for some regions (the usual wind tunnels like Wellington/Cook Strait and through the Southern Alps) but peaks aren't as high as Tuesday.
"Wellington has a bit of a windy week ahead but really only Tuesday has the severe gale risk."