Households across rural Timaru are being asked to conserve water as flooding has begun to contaminate drinking supplies.
The South Canterbury province has been inundated with rain for two weeks and is now facing a further 100mm from Tuesday afternoon through until midnight.
Timaru district council services manager, Ashley Harper, says town services are coping so far, but the Pleasant Point area has just started having trouble with its waste system.
He says ground water has flooded the sewerage system and raw sewage has now affected five properties in the township.
Mr Harper says rural residents are also having to contend with slips and subsidence under roads.
MetService has issued a severe weather warning, saying the rain is not expected to let up until Wednesday morning.
Forecaster Hordu Thordarson says between 40 and 90 millimetres of rain is expected to fall, which could worsen the flooding that has already inundated Otago and Canterbury.
People are being advised to be careful near rivers that could rise rapidly.
The Waitaki District Council says its priority is to reopen the 25 flooded roads around rural properties, inland from Oamaru and Palmerston. Two days of rain closed the roads for the third time in two weeks.
Waitaki Civil Defence says a handful of farmers are likely to be isolated for up to three days as flooding closes rural roads across the region.
Federated Farmers says farmers should survive being cut off for that long without much difficulty, though they may have trouble finding safe ground to move stock to if they are caught between rising waters and heavy snow in the high country.
In Christchurch, contractors are racing to mop up flooded low-lying areas in the east of the city and clear stormwater drains before the arrival of more heavy rain forecast for Tuesday.
Debris from overnight slips is also being cleared on the Port Hills, at Lyttelton and on Banks Peninsula.
Most roads closed on Monday are now open but motorists should watch for enlarged potholes and cracking and the Transport Agency is urging drivers to take care due to water on SH1 from Christchurch to Rakaia and SH75 from Christchurch to Birdlings Flat and from Barrys Bay to Akaroa.