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MetService has warned Dunedin and Oamaru residents to be prepared for heavy rain this weekend.
A ridge of high pressure over the country is expected to move east on Friday as a front moves on to the South Island from the Tasman Sea.
A large low is expected to develop on this front over the north Tasman Sea late Friday and Saturday, spreading rain and easterlies over the North Island, and causing the front to stall over central New Zealand.
At present, there is some uncertainty about the future movement and depth of the low.
A MetService spokesman said there was moderate confidence of heavy rain in Fiordland and southern Westland on Friday; and from Saturday to Sunday, there was a possibility of warning amounts of rain falling in eastern parts of Otago.
The low is expected to move slowly northeastwards during Sunday and Monday, with rain and strong easterlies affecting much of the North Island and upper South Island.