Why is Tasman getting so much rain? Subtropical weather pattern fuels third deluge

Tasman and Nelson in New Zealand’s South Island have been battered by a third severe rainstorm in just over two weeks, with local authorities declaring a state of emergency. Meteorologists are also warning that the pattern could continue into the coming months.

Once known for its sunny climate, the region has instead faced relentless rain, with homes evacuated and streams and rivers threatening to flood.

Experts say the intense and repeated rainfall is the result of a specific arrangement of weather systems, one that’s unusual, but not unprecedented for the region.


“We are so saturated,” said Tasman Mayor Tim King to RNZ. “The groundwater is literally already at the ground level. So every bit of rain that falls runs off… it’s fair to say I’m pretty concerned.”

According to Earth Sciences NZ, which includes NIWA and GNS Science, the current pattern involves atmospheric rivers, long, narrow bands of moisture-rich air, streaming in from the sub-tropics and tropics.


These systems funnel warm, wet air directly into the northwest of the South Island, bringing heavy, prolonged downpours.

What is the reason for flooding?

Chris Brandolino, principal scientist of forecasting and media at Earth Sciences NZ, explained that the current conditions are a classic recipe for flooding in Nelson and Tasman.

“When there's a deep depression in the eastern Tasman Sea and a large anticyclone east of New Zealand, it creates a strong northerly flow of moist air,” he said. “That’s what we’re seeing now.”

The MetService pressure map for Friday, July 11, showed precisely this setup. The northern regions of the South Island are particularly exposed to such northerly systems while being sheltered from the south and west.

“This kind of pattern increases the odds for big rainfall events,” Brandolino said. “We’re in one right now, and it looks set to continue.”

Is it the region’s usual climate?

While such events are not unheard of, they’re certainly not common for the area. The Waimea Plains and greater Tasman region are usually protected from severe weather by surrounding hills and mountain ranges, enjoying mild, less windy conditions than most of New Zealand.

But the current back-to-back storms have overwhelmed local catchments, with rainfall setting records since late June.

The saturated ground means new rain has nowhere to go, quickly swelling rivers and flooding roads and homes.

More rain likely in the coming months

The pattern, which began in late June and led to states of emergency across Tasman, Nelson, and Marlborough, is unlikely to ease anytime soon.

The NIWA Climate Outlook for July to September warns of further tropical and sub-tropical influences, increasing the risk of heavy rainfall and flooding.

“There’s a high likelihood for these kinds of low-pressure systems to keep affecting New Zealand,” said Brandolino. “This may mean more active weather toward the end of July and even into early August.”

Locals brace for continued impacts

With saturated soils and full waterways, even moderate rainfall could now pose serious risks. Emergency management authorities and local councils continue to monitor the situation, urging residents to remain prepared and alert.

“This is the third event in just over two weeks,” said King. “The community is resilient, but we’re exhausted—and still in the thick of it.”