MetService is warning of more "intense bursts" of rain, high winds and thunderstorms across much of the country, including areas already sodden by recent rain.
The warm, wet weather of the past week has moved west but is due back across the country on Tuesday, bringing more heavy rain to northern and western areas as it tracks slowly to the east.
"We've had really mild temperatures the last few weeks or so, however things are changing" - MetService communications meteorologist John Law
A severe thunderstorm watch has been issued for Northland, with MetService warning the region could see localised torrential rain in excess of 40mm/h tomorrow. Tornadoes were also a possibility, the forecaster said.
Orange heavy rain warnings are in force for Northland, Auckland (including Great Barrier Island and Coromandel Peninsula), Mount Taranaki, Tasman (from Motueka westwards), the Grey and Westland districts, the headwaters of the Canterbury lakes, the rivers around and south of Arthur's Pass, and Fiordland (north of Doubtful Sound).
Heavy rain watches are also in place for North Taranaki, Bay of Plenty, Marlborough (about and north of Awatere Valley, as well as Nelson and Tasman south and east of Motueka), Buller, and the headwaters of the Otago lakes and rivers.
MetService communications meteorologist John Law told Morning Report the weather system was likely to affect the many of the same areas which had already seen a lot of rain in recent days.
The West Coast could see another 300mm of rain over the next day or so, he warned, but Nelson, Tasman and possibly even parts of the Bay of Plenty could also be hit again.
"That's a fairly huge amount of rainfall on top of what we've seen in the last few days."
The weather meant there was risk of surface flooding and possibly more slips in some areas, he said.
"There could be some thunderstorms mixed in with it as well, so some pretty intense and heavy bursts of rain coming through."
Law said the current weather was being driven by sub-tropical air being dragged in from the north but a cold snap was predicted later in the week.
"We've been so warm over the last week or so ... we've had really mild temperatures the last few weeks or so, however things are changing."
He said once the current system pulled away to the east, the winds would change from a northerly to a southerly which would drop temperatures across the country, possibly bringing snowfall to some higher South Island altitudes.
There would be "a real contrast in those temperatures at the start of the week, compared to the end", he said.
Westland on standby
West Coast authorities are preparing for the expected heavy rain, with Westland mayor Helen Lash telling Morning Report the potential for slips was a particular concern.
"Our advisement will be for people to only travel if absolutely essential" - Westland District mayor Helen Lash
"We've got massive catchments to every river area in Westland and South Westland in particular and if you get that heavy rain up there, that's going to exacerbate what's falling at the coastal area as well," she said.
"It's all compounding and it can become very, very problematic very, very quickly."
"Everybody's on standby, everybody has been called in to work through this, all agencies are meeting."
Lash said the New Zealand Defence Force had resources and staff on standby in Christchurch should they be needed to help the district, and neighbouring regions were on alert too.
"The word's got out to the communities and we will keep getting that word out to communities through the day and our advisement will be for people to only travel if absolutely essential, but preferably stay off the road when this rain comes."
She said bridges in the district appeared to have held up well in recent weather events but the potential for land movement was a concern.
"It's the slips that'll be the problem for us, they'll come down and they'll come down quickly; they'll come down with a lot of debris and they'll potentially close the roads."
Heavy rain causes sewage overflows in Tasman Bay
A rāhui is in place in Tasman Bay until further notice, after heavy rain caused numerous sewage overflows and sustained runoff into the ocean in the top of the South Island.
The rāhui was implemented by the Iwi Emergency Management Rōpū, with the support of the eight iwi of Te Tauihu.
It covers all coastlines, river mouths, and floodwaters from Te Matau (Separation Point) to Raetihi (Cape Soucis).
The rāhui prevents swimming and entering the water, the gathering of seafood and kai including shellfish. In-shore fishing and the gathering of crayfish, crab, kina and paua are also prohibited.