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Heavy rain and severe gales are on the way for the southern regions, and possible heavy snow for southern Fiordland tonight.
A MetService spokeswoman said a low and its associated fronts were expected to move onto the South Island from the Tasman Sea later today.
"The low should move away to the southeast early tomorrow while the associated fronts, followed by a cold southwest change, move northeast across New Zealand.
"This weather system is forecast to bring heavy rain and strong winds to many parts of central and southern New Zealand, and possible heavy snow to southern Fiordland."
Heavy rain and severe gale warnings and watches were already in force for many places, and a heavy snow watch was now in force for Fiordland, south of Te Anau, she said.
Between 6pm tonight and 6am tomorrow, up to 140mm of rain is expected to accumulate in the headwaters of the Otago lakes and rivers, and up to 100mm within 15km east of the divide.
Peak rates of up to 20mm per hour could fall, especially near the main divide.
In Fiordland, up to 100mm of rain could fall between 4pm today and 3am tomorrow, with peak rates of up 20mm per hour.
"Heavy rain may cause streams and rivers to rise rapidly.
"Surface flooding and slips are also possible, and driving conditions may be hazardous."
Northwest gales were also expected to be severe at times, gusting up to 120kmh in exposed parts of Fiordland, Southland and Otago (apart from North Otago), between now and 11pm today.
"Strong wind gusts could damage trees, powerlines and unsecured structures," she said.
"Driving may be hazardous, especially for high-sided vehicles and motorcycles."
A second colder southwest change was expected to spread up the country from late Tuesday into Wednesday, bringing snow to even lower levels.
Warning amounts of snow are not expected with this second cold change at this stage. However, snowfall is expected to affect many South Island roads and passes and may cause stress to livestock, she said.
"People are advised to keep up to date with the latest forecasts," she said.