The low pressure system currently over New Zealand is so large it would engulf Australia, WeatherWatch says.
"While bigger doesn't always mean more aggressive, as the energy is spread over a much large area, it does explain why the nation is experiencing scattered rough weather and why this has been going on for five days now," the forecaster said in a statement.
"On Thursday the low weakens and by Friday it's gone to our east, as another low forms to our west, and 24 hours after that may also be getting close to the size of Australia as it tracks towards us."
Big lows have bigger reach, meaning they scoop more air from north and south of the country. This results in bigger weather changes, such as the thunder, warmth and snow this week.
"Big lows can also have large areas of dry, calm, sunny, weather in the middle too."
MetService is warning of more rain, squalls, strong winds, "a nasty band of thunderstorms" and even coastal tornadoes today.
For Auckland, there is a moderate risk of thunderstorms about midday, which are likely to be accompanied by heavy rain, hail and strong winds up to 110km/h.
There is also a risk of severe thunderstorms from the Waikato to the Kapiti Coast and possibly small coastal tornadoes.
Weather has been wreaking havoc this week, causing road closures and multiple accidents in the past couple of days.
It's also been hitting Kiwis in the pocket, with $3.2 million in additional electricity usage consumed over the weekend during the cold snap.
The biggest surge in power use was in the lower North Island, followed by the upper South Island and upper North Island, according to Electricity Authority figures.
Powershop general manager Mark Soper said people budgeting for power will need to put more aside to account for the current cold weather.
"The cold snap will stick around for the rest of the week according to Metservice and even after that temperatures are only forecast to rise slightly.
"Households are about to be hit with their first significant power costs for the year. Planning for these extra costs now is important to avoid financial stress or debt later on."
Meanwhile, a small magnitude 2.7 earthquake shook White Island just after 7am this morning. It was 6km deep, and centred within 5km of White Island.
The island's volcano erupted on April 27. The eruption would have killed anyone standing close to the floor of its Crater Lake, GNS Science volcanologist Brad Scott later said. Fortunately, the eruption happened late at night and no one was on the island when a surge of ash was thrown across the crater floor.
Mr Scott said it was highly possible there would be further eruptions.
"White Island can and does continue to intrigue us, providing us rich scientific data that increases our understanding of volcanoes in New Zealand and around the world.
"We aren't entirely sure what the volcano will do next."
Main centres outlook
Periods of rain, possibly heavy. Strong southwest change. High 20C.
Auckland: Periods of rain, possibly heavy. Strong southwest change. High 18C.
Tauranga: Rain, possibly heavy, and strong northwest winds, easing in the evening. High 17C.
Hamilton: Rain with a chance of thunder. Rain eases in the evening with a southwest change. High 16C.
Wellington: Rain clearing to become fine in the afternoon. Gale northwest winds ease. High 15C.
Christchurch: High cloud. Northeast for a time. High 12C.
Dunedin: Increasing high cloud. Northerlies. High 12C.