Part of New Zealand’s South Island has become cut off after days of torrential rains washed away roads, forced the evacuation of 2,000 people and saw the country’s MetService issue its first ever red weather warning.

The flooding across the flat agricultural plains of Southland, in the island’s southernmost end, came a day after hikers and tourists were evacuated from another part of the region, Fiordland, where more than a metre of rain had fallen in less than three days.

Lewis Ferris at New Zealand’s MetService said the storms had generated its first red warning, a new alert the agency created in May last year. “The difference between the orange and red warnings is the impact on people,” he said. “We saw a road washed out, we saw people isolated. There was a real risk to communities given the impact on the roading network.”

In Southland, reports emerged of farmers trying to herd livestock to higher ground using a jetski or kayak. Neville Cook, a civil defence official, said: “There will be stock losses. Even with prior warning, there’s really nowhere they can go.”

In the township of Mataura, the floodwaters are inching closer to a paper mill where a chemical called ouvea premix is stored, which could release toxic ammonia gas if it comes into contact with water, authorities said. The gas – harmful to people and the environment – will likely be released into the floodwaters, though emergency management authorities hope it will be diluted by the water.