Northland and parts of Taranaki now face heavy rain warnings as MetService expands alerts with a "significant weather event" forecast to bear down on large areas of the country in the coming days.
MetService said a trough embedded in a humid northerly flow was forecast to bring prolonged heavy rain to northern and western parts of the South Island from Sunday, before spreading into parts of the North Island on Monday.
Fresh orange heavy rain warnings were issued for Northland and Taranaki Maunga, while new watches were enacted for the Coromandel Peninsula, Bay of Plenty, Rotorua and Taupō.
The most severe conditions were expected for the upper South Island, with the Tasman District northwest of Motueka remaining under an orange heavy rain warning into early Tuesday.
Between 350 to 400mm of rain was expected, with the possibility of more than 500m above the Kahurangi ranges. There remained a moderate change of upgrading to a rare red level warning.
Orange heavy rain warnings remained in place for the Richmond and Bryant ranges, including Nelson city, the ranges of Buller, Westland south of Hokitika, Fiordland north of Charles Sound and the headwaters of the Canterbury Lakes.
MetService meteorologist Paris Marshall told 1News a "slow, stationary front" would cause heavy rain to batter the South Island.
"That stationary front hanging around the South Island at the moment is very, very slow. So that rain will just continue to pour down on the West Coast," she said.
"It's definitely going to be significant."
Marshall said while the West Coast was accustomed to periods of intense rainfall, the impacts could be felt more strongly in areas such as Tasman and Golden Bay, where significant rain events were less common.
Further north, Taranaki Maunga was forecast to receive between 250mm and 300mm of rain, while Northland could expect between 80mm to 130mm of rain and up to 200mm in localised areas.
MetService warned the heavy rain across the country could cause streams and rivers to rise rapidly, with surface flooding and slips potentially making for hazardous driving conditions.
People in affected areas should stay up to date with forecasts and prepare before the heaviest rain arrived, Marshall said.
“Clear your drains and gutters, that is very important. That water needs somewhere to go, so that's the best thing you can do to stay prepared."
Full list of updated weather warnings and watches:
Heavy Rain Warning - Orange
Northland - 20hrs from midday Monday to 8am Tuesday.
Taranaki Maunga - 33hrs from midnight Sunday to 9am Tuesday.
Headwaters of the Canterbury Lakes and Rivers about and south of Arthur's Pass - 13hrs from 9am to 10pm Sunday.
Fiordland north of Charles Sound - 1hrs from 9am to 8pm Sunday.
Tasman District northwest of Motueka - 41hrs from 9am Sunday to 2am Tuesday.
Ranges of Buller - 36hrs from midday Sunday to midnight Monday.
Richmond and Bryant Ranges, including Nelson City and the Rai Valley - 40hrs from noon Sunday to 4am Tuesday.
Heavy Rain Watch
Coromandel Peninsula - 21hrs from midday Monday to 9am Tuesday.
Bay of Plenty and Rotorua - 21hrs from 6pm Monday to 3pm Tuesday.
Taupo - 18hrs from 9pm Monday to 3pm Tuesday.
Parts of Tasman District not covered by warnings - 40hrs from midday Sunday to 4am Tuesday.
Headwaters of the Otago lakes and rivers - 11hrs from 9am to 8pm Monday.
Strong Wind Watch
The Canterbury High Country - 8hrs from 9am to 5pm Sunday.
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