"We are in for a wet weekend," James said.
MetService has issued heavy rain warnings for parts of Westland from 8am to 11pm tomorrow with up to 100mm forecast in the ranges.
Heavy rain is also expected about northwest Nelson and northern Fiordland, and Taranaki and Tongariro National Park in the North Island.
On Saturday and Sunday another low over the western Tasman Sea is expected to maintain the moist north to northeast flow over New Zealand.
WeatherWatch analyst Philip Duncan said the North Island could be in for some very heavy rain.
"The main threat for New Zealand is this weekend in the upper half of the North Island as very heavy bands of rain slowly track eastwards."
Converging sub-tropical winds - northwesterlies from the Tasman low and northeasterlies from the departing high - would create a very heavy, potentially torrential, narrow slow moving band of rain, he said.
"[It] will cross over the northern half of the North Island from Saturday afternoon to Monday morning at this stage."
MetService said there was potential for further heavy rain from Fiordland to northwest Nelson.
Rain could be heavy about western areas in the North Island on Saturday and Sunday, and Bay of Plenty and the Gisborne ranges on Sunday and Monday.
Temperatures are forecast to stay fairly mild in the North Island, with Auckland forecast to hover around 20C through to mid next week.
The South Island however is in for a cool southerly change on Saturday, with Christchurch dropping to a maximum of 14C, and just 12C on Sunday.
Temperatures are forecast to remain in the low teens across the South Island through next week.
Today, a ridge of high pressure is keeping the weather relatively settled throughout the country.
This morning's fog in Auckland is forecast to clear late morning for a fine day, with a high of 19C.
Similar conditions are forecast across the North Island today and tomorrow until late afternoon.
In the South Island there might be some isolated showers in the afternoon, before the low arrives from the Tasman.
Today's weather
• Whangarei
Becoming cloudy, could also be some morning fog. Northeast breeze. 20C high, 14C overnight.
• Auckland
Fine with high cloud, but areas of morning low cloud or fog. Northeast breezes. 19C high, 15C overnight.
• Hamilton
Morning low cloud or fog, otherwise fine with high cloud. Light winds. 20C high, 11C overnight.
• Tauranga
Fine with high cloud, but areas of morning low cloud or fog. Northeast breezes. 19C high, 14C overnight.
• New Plymouth
Fine with high cloud. Northerly breezes. 18C high, 14C overnight.
• Napier
Fine, high cloud. Light winds. 20C high, 11C overnight.
• Wellington
Cloudy at times. Northerlies. 17C high, 13C overnight.
• Nelson
Becoming cloudy, few showers from afternoon. Northerlies. 17C high, 13C overnight.
• Christchurch
Fine with high cloud. Northerlies developing. 17C high, 9C overnight.
• Dunedin
High cloud. Possible light rain around midday. Northerly. 18C high, 10C overnight.
