MetService has issued an Orange Warning for heavy rain in Fiordland tomorrow.
A front is expected to move on to Fiordland tonight and then stay slow-moving during Sunday.
A MetService spokesman said between 120mm and 160mm of rain could accumulate between 9am on Sunday and 9am on Monday, with peak rates of 10mm to 15mm per hour.
There would also be periods of heavy rainfall in the headwaters of the Otago lakes and rivers, between 4pm on Sunday and 11am on Monday.
"Rainfall amounts may approach warning criteria within 15km east of the main divide."
He said the front should move southeastwards on Sunday night.
"A couple of cold fronts, preceded by strong northwesterlies, are expected to move northeast across the South Island from the Tasman Sea on Monday, followed by cold southwesterlies on Tuesday.
"These fronts should bring periods of heavy rain to western parts of the South Island.
"A heavy rain warning is now in force for Fiordland, and a heavy rain watch is in force for the ranges of southern Westland and the headwaters of the Otago lakes and rivers.
"Heavy rain may cause streams and rivers to rise rapidly. Surface flooding and slips are also possible and driving conditions may be hazardous."
He advised residents to keep up to date with the latest forecasts in case parts of the watch were upgraded to a warning or further areas were added.
In addition, there is moderate confidence that northwest gales will become severe in exposed parts of Fiordland Lakes, Southland, Otago and the south Canterbury Plains during Monday, Wednesday and again on Thursday.
