The West Coast, sodden after two days of heavy rain that washed out a bridge and cut communications, is being warned to expect another deluge.
The MetService has issued a weather watch for the region on Wednesday, predicting it will get a drenching from an active front.
"The western and alpine areas of the South Island may see another period of heavy rain, which may pose problems to those areas affected by the wet weather of last week," said spokesman Ramon Oosterkamp.
A 40-metre section of State Highway 6 on the approach to the one-lane bridge at Harihari was washed away by the flooded Wanganui River last Wednesday, making the road impassable and forcing lengthy detours for motorists.
The New Zealand Transport Agency diverted floodwaters away from the damaged section to allow work to begin rebuilding the road and says it hopes to reopen it tomorrow.
Thunderstorm warning for Canterbury, Otago
The headwaters of Canterbury and Otago rivers and lakes are expected to get rain as well, while eastern parts of the country will have warm temperatures and gale northwesterlies to deal with.
Hailstones the size of 20 cent pieces could fall in North Otago and Canterbury this afternoon as a front passes over the regions.
The MetService has issued a severe thunderstorm watch for Oamaru north to Amberley.
It says there is a moderate risk of thunderstorms which were likely to produce large hail 20 millimetres or more in diameter, accompanied by heavy rain and wind gusts of 110km/h or more.