Regional councils in Otago and Canterbury have issued flood warnings and are monitoring heavy rain expected to intensify overnight on Monday.
MetService says the hills between the Rangitata River and Dunedin could get up to 250 millimetres by midnight on Tuesday, and another 100mm on Wednesday.
The front is predicted to stall over South Canterbury and North Otago on Tuesday, bringing two days of heavy rain and gale-force winds.
Civil Defence says Canterbury and Otago are expected to bear the brunt of the storm and is warning residents near streams and rivers to stay alert.
Heavy rain is also expected for parts of Bay of Plenty, Tongariro, National Park, northern Gisborne, Central Hawke's Bay and Wairarapa.
Canterbury Civil Defence officer Andrew Howe says much depends on how quickly the rain falls, but a similar weather pattern in 2008 caused floods, slips, road and railroad closures in North Canterbury.
The Timaru District Council is on flood watch. Council spokesperson Phillip Howe says the amount of rain forecast could cause the kind of serious flooding last seen in 1986. People in low-lying places north of Timaru are warned to prepare to evacuate.
Mr Howe says the council has activated its Emergency Operations Centre and staff will be on duty overnight on Monday to monitor the situation. South Canterbury residents in vulnerable areas should listen to their radios for information.
Severe flooding predicted in Otago
The Otago Regional Council warns heavy rain overnight on Monday and Tuesday is likely to cause severe flooding in all catchments north of the Taieri River and many roads could be under water for most of the week.
They include: State Highway 1 from Waitati to Kakanui, State Highway 83 from Oamaru to Omarama and State Highway 85 from Palmerston, inland.
The council says the storm was starting to develop late on Monday night, with 30mm falling in the Kakanui catchment.
It says farmers should move stock from low-lying areas, householders should check drains and spouting for blockages, and boaties should check their moorings.
Otago Regional Council manager Matt Hickey says the region badly needs rain after a dry summer - but not that much in one hit. He says predictions of up to 350mm by Wednesday would be the highest rainfall on record for Otago in so short a time.
River levels rise in Bay of Plenty
Council staff are on stand-by for flooding in the Bay of Plenty, as river levels rise after heavy rain.
Regional council Environment Bay of Plenty says some of the heaviest rainfall has been in the Mangorewa-Kaituna River catchment. It says the Kaituna River has been rising steadily on Monday and farmers should move stock to higher ground.
About 100mm of rain has fallen on Tauranga and the western Bay of Plenty since 9am, while 126mm had fallen in Matataa in the east.
More heavy rain is forecast, but is expected to ease on Tuesday.
