"A large part of both islands get lots of rain."
MetService has put out a severe weather watch for Waitomo, Waikato and the Coromandel Peninsula.
The MetService forecast predicts 20 to 40mm of rain could accumulate in these regions during Sunday. But this could be higher in localised places. Possibly 40 to 60mm could fall as the risk of thunderstorms could increase the rainfall in some areas.
"This amount of rain could exacerbate the already saturated conditions in these areas, causing rivers and streams to rise further, with possible surface flooding and slips," states the forecast.
Weather Watch weather analyst Philip Duncan said Saturday won't be too bad but tomorrow could see some places get up to 80mm of rain. But while it will be wet, it won't be cold. The weather is coming out of the Australian tropics so the weekend will be warmer than usual, Duncan said.
He said the Far North is starting to get the first front of the next weather system which will see showers increasing throughout the day and into tomorrow.
The slow moving front brings a moderate to high risk of thunderstorms which could see some localised flooding tomorrow.
"Potentially very localised bursts of rain could cause flooding or slips.
"It could be quite a solid, slow downpour."
Lee thinks the best part of the country will be Christchurch as it should remain the driest with a nice high of 21C forecast for tomorrow.
The worst part will be the west coast of the North Island with areas from Raglan down to New Plymouth looking pretty wet for the next 24 to 48 hours.
Auckland and the Bay of Islands are set to experience showers today that will turn to heavy rainfall and potentially a thunderstorm tomorrow.
Weather Watch forecasts a shift from the easterlies September has seen to strong gale westerlies that will kick in this week. The westerly will blow away the La Nina-like weather away for a time.
Duncan predicts this month could be warmer than average which "ties into the theme of 2016".
"September and October are always crazy chaotic weather months that are all over the place."
Main centre forecasts
Whangarei
Showers turning to late rain. Northerly winds and a high of 20°C.
Auckland
Cloudy period and showers. Northeasterly winds and a high of 18°C.
Hamilton
Showers that will increase in the afternoon. Northerly winds and a high of 19°C.
Tauranga
Showers that will become more frequent in the afternoon. Northeasterly winds with a high of 17°C.
Wellington
Cloudy periods with a few showers possible. Northerly winds and a high of 16°C.
Christchurch
Sunny day with evening cloud. Northeasterly winds and a high of 19°C.
Dunedin
Cloud in the morning and night. Northeasterly winds and a high of 15°C.
Queenstown
Sunny spells and evening showers. Northwesterly winds and a high of 19°C.
