The Buller District including Westport may be in for a pounding this week as an "incredibly persistent" northerly airflow hits.
There's an orange rain warning in force for Buller, Westland and the Tasman district west of Motueka from early tomorrow morning.
There's also a heavy rain watch for the rest of the Tasman district, including Nelson, the Marlborough Sounds, and the Bryant and Richmond ranges, including the Rai Valley.
Some areas may receive more than 500mm of rain.
"This will be a significant event," MetService said on Twitter.
Buller Civil Defence is giving people sandbags and has urged people to have a bag of belongings ready to go if they have to leave their homes.
MetService meteorologist Lewis Ferris said for the north-western section of the South Island it was possible that the orange warning might be upgraded to red once the MetService has spoken to the regional council and Civil Defence later today.
He said a northerly flow that was approaching was looking "incredibly persistent". It had the potential to deliver some "elevated rainfall", perhaps higher than the current prediction of 10mm to 20mm of hourly rainfall.
Those living in Westport and the Buller district were most at risk.
"People need to remain up with our forecasts... It's really going to turn to custard as we move into the week and this long period of heavy rain could even see a little bit more rain as we head into Friday..."
Ferris urged people to follow the advice of local authorities.
While Wellington - recently affected by a cluster of slips after weeks of rain - would receive some passing rain it did not look as though it would be heavy.
Waka Kotahi has warned motorists to be prepared for road closures because of heavy rain forecast on the West Coast.
The transport agency said there was an increased risk of fallen trees, slips and rockfall because there has already been so much rain this winter.
'I'm very concerned' - Buller mayor
Heavy rain forecast for the Buller District has the potential to cause flooding similar to that which forced people from their homes in February and July last year, the mayor warns.
Buller mayor Jamie Cleine said 300 to 500 millimetres of rain was forecast to fall in the ranges and 150 to 250 millimetres in the lower reaches, before easing on Friday.
While he was yet to get regional council advice about the flood risk, Cleine said Civil Defence was preparing to establish an emergency operations centre in Westport.
"It's fair to say I'm very concerned," he said.
"Certainly these sort of numbers would put it on the scale not dissimilar to July or February, just depending on the distribution."
Cleine said it was too early to say whether people might need to evacuate their homes, some of whom have only just returned after flood repairs.
"There are people literally moving back into houses every week that have just finished their refurbishment and renewed all their household goods. It increases anxiety for everybody," he said.
The council was handing out sandbags in Westport, Reefton, Granity, Hector, Ngakawau, Mokihinui and Seddonville, and pumps were being sent to flood-prone parts of Westport.
People were being urged to have a bag of belongings ready in case they have to leave at short notice - including food and water - and a plan in place for their pets.
West Coast Emergency Management director Claire Brown urged people to stay up-to-date with warnings issued over the next few days.
"This could be a serious event and it is important that people know what may happen as the event unfolds," she said.
Anyone feeling anxious about the forecast could get help at the Buller community hub in Palmerston Street, Brown said.
RNZ
