Thunderstorms and northwest gales were also possible in parts of the North Island, Mr Holland said.
"The most severe weather will be Saturday morning. Later on in the day it will still be windy but not as severe.
"Sunday will be showery with rain over the North Island and northwest gales for a time over the northern half of the North Island."
Mr Holland said there wasn't a "place to be this weekend". "Parts of the South Island won't be as windy but it's all not great."
Meanwhile, WeatherWatch forecaster Philip Duncan said the "incredibly fast development" of a low meant a rough weekend for some places.
"While a large low often means the rain clouds get fractured and broken up, the low has enough energy to bring localised flash flooding to some areas," he said.
Mr Duncan expected to see very wet easterlies into Otago and parts of Canterbury on Sunday, along with wet west to northwest winds in the North Island.
"The flip side means the west of the South Island may be mostly dry on Sunday but heavy rain could still form over the dry eastern North Island as a colder southerly sweeps in," he said.
"Rain may also become heavy in Dunedin and coastal Otago this weekend -- with set-in rain lasting a day or so, so those concerned with flooding may want to keep a close eye on the movements of this low."
Unsettled weather was likely to continue into early next week, according to MetService.
MAIN CENTRES OUTLOOK
Whangarei:
Today: Sunny spells with possible showers. Westerlies turn northerly. High 21C.
Saturday: Heavy early rain then showers. Strong gusty northwest. High 22C.
Sunday: Few showers then rain. Northwesterlies. High 20C.
Auckland:
Today: Sunny spells with possible showers. Westerlies turn northerly. High 19C.
Saturday: Heavy early rain then showers. Strong gusty northwest. High 20C.
Sunday: Few showers then rain. Northwesterlies. High 19C.
Tauranga:
Today: Fine spells with possible showers. Westerlies turn northerly. High 18C.
Saturday: Heavy early rain then showers. Strong gusty northwest. High 20C.
Sunday: Few showers then rain. Northwesterlies. High 19C.
Hamilton:
Today: Cloudy periods and a few showers. Westerlies turn northerly. High 17C.
Saturday: Periods of rain, some heavy and possible thunder. Northwest. High 19C. Sunday: Showers, some heavy. Northwest easing. High 18C.
Wellington:
Today: A mostly sunny day. Rain at night. Northerlies. High 16C.
Saturday: Rain, possibly heavy and thundery. Strong gusty northwest. High 16C.
Sunday: Occasional showers. Northwest changing southerly. High 14C.
Christchurch:
Today: Mainly fine. Patchy drizzle and northeast develop at night. High 12C.
Saturday: Rain, heavy at first. Cold southerlies. High 11C.
Sunday: Occasional showers. Cold southwesterlies. High 9C.
Dunedin:
Today: Fine then cloud developing in the afternoon. Light winds. High 12C.
Saturday: Rain developing in the morning. Cold southwesterlies developing. High 10C.
Sunday: Occasional showers. Cold southwesterlies. High 10C.
- Source: MetService