New images show just how large and forceful the low-pressure system behind the severe weather in Canterbury is.
A red-alert deluge is swamping the region with widespread flooding forcing Timaru, Ashburton, and Selwyn to declare a state of emergency on Sunday.
The wet weather is causing dangerous river conditions and extensive flooding across the region. Slips and floodwaters are disrupting travel, making some roads impassable and "possibly isolating communities", according to MetService.
The broad low behind the carnage is sitting across Aotearoa. The centre of it is up over the North Island and the tail end is causing wild weather in the South Island.
WeatherWatch has shared 10 images of the low-pressure system which they say highlights the "size and forces behind what is happening".
The first image shows airflows converging to cause heavy rainfall in Canterbury.
The second is a Himawari satellite picture from 4pm Sunday taken as darkness approaches the country from the east.
"Rain will continue all night and across a big part of Monday too, but should ease back slowly," WeatherWatch says.
"Also, note the big southerly in the Tasman Sea, it almost looks like a mushroom cloud of southerlies pushing out into eastern Aussie."
The following are a couple more shots from the Himawari satellite.
This next one shows a closer view of clouds to the left of Aotearoa.
"The big south-east airflow makes for stunning clouds as the air moves over the South Island ranges," WeatherWatch explains. "It's a reverse of the Canterbury nor'wester which brings rain to the West Coast and often stunning cloud displays in the dry east."
The next image shows the 24-hour Accumulated Rainfall for Sunday. The large spiral low can be seen clearly. So can the blocking nature of the Southern Alps which is pushing up the rainfall totals in Canterbury.
The next is MetService's rain accumulation map based on rain radar data.
This air pressure map shows just how wide the low is.
"This is a huge low," WeatherWatch says. "It stretches 3400km wide (west to east). Australia is 4000km wide! NZ is 1600km long (north to south)."
"In saying that, the low over NZ isn't really stormy, it's just big with a lot of rain in one place (made heavier by the Southern Alps). The low in the South Pacific towards South America is a real storm - it's many times larger, deeper and windier."
