Civil defence personnel on the West Coast have set up an emergency operations centre in Greymouth as they monitor the weather that is expected to worsen.
A heavy rain warning has been issued for Westland for the next two days, and a watch is now in place for other parts of the lower South Island.
NZTA has also advised drivers to take care as today's heavy rain has caused surface flooding on State Highway 6, in the Franz Josef to Haast Pass areas.
MetService said some local rain gauges on the West Coast showed up to 150 millimetres of rain had fallen in the 24 hours to mid morning, in areas of the western ranges.
Civil Defence was warning whitebaiters of potential flooding near waterways, but said the catchments were holding out well so far.
Last month, a nine-year-old Christchurch girl was swept away while whitebaiting at the mouth of the Hokitika River.
The West Coast CDEM regional manager Mark Crowe said the centre had been set up at the group's office in Greymouth as a precaution.
"We have established an EOC here, just to keep an eye on things in case it does take a turn for the worst suddenly, so we are prepared for that but there's no great alarm at the moment."
MetService said the weather system currently battering the West Coast is likely to affect the entire country by later in the week.
MetService forecaster Lewis Ferris said more rain is likely, with 200 to 250mm more rain in the next 30 hours in Westland and south of Otira.
Mr Ferris said further warnings are now out for other parts of the lower South Island.
"As we move into Thursday and Friday the front gets moving again and sweeps across the entire country," he said.
"We have issued a watch for heavy rain for the coastal Otago and Southland regions because they have a lower threshold for (heavy) rain."
Mr Ferris said that made these regions more prone to flooding than the West Coast catchments.
The next MetService update is due later this evening.