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An active front is moving east over the South Island, and a low pressure system is forecast to deepen just south of the South Island tomorrow.
A MetService spokeswoman said the front and low system are expected to bring periods of heavy rain and gales to exposed areas.
A heavy rain warning is in place from about Aoraki/Mt Cook northwards.
"Expect a further 80 to 120mm to accumulate on top of what has already fallen, but 20 to 40mm further south. Peak rates of 20 to 30mm/h especially in the north," the MetService said.
"Lesser amounts in the south due to the passage of the front.
"Heavy rain may cause streams and rivers to rise rapidly. Surface flooding and slips are also possible and driving conditions may be hazardous."
A heavy rain watch is in place for parts of South Canterbury, south of about Tekapo to Timaru, including parts of Otago, north of Wanaka to Middlemarch.
Between 8pm tonight and 5am tomorrow, rainfall accumulations may approach warning criteria.
"People are advised to keep up to date with the latest forecasts in case part of these watches are upgraded to a warning or further areas are added."