MetService has issued a rare red warning for heavy rain in Westland and Buller, advising "extreme rain accumulations" will cause disruptions across the regions.
The heavy rain warning comes into place from Wednesday morning until Thursday night but MetService states the warnings could be extended.
"The West Coast is one of the wettest parts of New Zealand, but this system is far from a normal wet day on the coast," meteorologist Lewis Ferris warns.
"Rain accumulations are forecast to approach 750mm about the ranges in a 42-hour period with 150 to 250mm near the coast."
The persistent heavy rain will also be accompanied by the risk of downpours in thunderstorms.
"This brings the potential of rain rates around 40mm/hr which would amplify the speed at which impacts are observed," MetService adds.
"Even though river levels are currently low the intensity of rain to come will mean these quickly rise and extreme caution is advised."
The level of rain means "significant impacts" such as flash flooding and landslips are expected, which will likely impact on roads.
MetService warns communities are at risk of being isolated with possible power and communications outages.
Neighbouring regions issued orange warnings for heavy rain are encouraged to be on alert too.
"While the biggest impacts are likely within the red warning area, bear in mind that surrounding regions will also need to be on alert as large amounts of rain falling near the peaks can cause rivers to rise," Ferris says.
This is the fifth red warning MetService has issued since the colour-coded system was introduced in 2019.
