Southern police are warning drivers to keep off roads if the predicted storm hits the district.
The storm was expected to bring 80mm to 150mm of rain to eastern Otago by Saturday morning, as well as gusts of up to 100km/h, according to Niwa.
A heavy rain warning is in place for Dunedin, with MetService predicting 150mm to 200mm to fall from midday on Friday to 6am on Saturday.
"It's a little bit of an unknown,'' Senior Sergeant Mark Donaldson said today. "But we are taking it seriously.
"We have got a bit of a focus on the roads in the next 24 to 48 hours with this weather bomb potentially hitting.
"People should drive to the conditions and don't go out if you don't need to."
An Otago Regional Council spokeswoman said the council would be keeping a close eye on river levels and coastal catchments over the next day and a half as the heavy rain hits.
"Rivers will rise rapidly and surface flooding is expected in low-lying areas, including South Dunedin," she said.
The Kakanui, Shag, Kauru and Waianakarua Rivers in North Otago as well as the Leith, Silver Stream and Taieri Rivers were all expected to rise swiftly, based on the current weather forecast.
As well as monitoring river levels, ORC staff were out checking pump stations to check that they are operating as expected.
Dunedin City Council infrastructure and networks general manager Ruth Stokes said the council was ''closely monitoring the situation'' as the storm approached.
''Staff and contractors will be making the appropriate preparations ahead of the predicted rain, such as making sure stormwater screens at pumping stations are kept clear of debris.''
Meanwhile, enjoy the weather in Dunedin today - because it's the calm before the storm.
Niwa climate scientist Nava Fedaeff said Dunedin should expect a high of 13degC today and that the mercury would stay above 8degC overnight.
The relatively warm weather was a ''precursor to the wild weather'' forecast to hit on Friday.
The warmth was from the northwesterly winds ahead of a front, which will bring rain to Otago, particularly to the east of the province.
Areas in eastern Otago should brace for 80mm to 150mm of rain by Saturday morning. The average rainfall for Dunedin in July is 57mm.
''Eastern Otago could get a month's worth of rain, maybe even double that, or more, in some places in less than 36 hours.''
The rainfall could cause flooding and slips, she said.
The areas set for a drenching were also ''in line'' for the ''strongest'' winds of the storm.
Gusts for coastal Otago and exposed areas could exceed 100kmh tomorrow, she said.
Areas further inland and sheltered from the prevailing southwesterlies would get lighter rainfall.
The winds and rain would ease on Saturday to make way for a dry Sunday, she said.
Jaffa Race plan
Cadbury World events manager Kylie Ruwhiu-Karawana said the Jaffa Race was scheduled in Baldwin St, Dunedin, tomorrow, as part of the Cadbury Chocolate Carnival.
''We'll wait and see and make a call on the day,'' she said about whether it would go ahead.
A reserve day had not been scheduled. The race had never been postponed during her eight years at the company.
''It has rained on some of the days - it rained quite heavily on race day in 2012 - but we've been quite lucky with the weather, but this could be the year it all changes.''
Rugby semifinals
Otago Rugby Football Union general manager Richard Kinley said the union would monitor the situation regarding the two Dunedin premier grade semifinals due to be played at Hancock Park, St Kilda, on Saturday.
''We just have to keep a watching brief on it and see what happens.
''There has been no alternative plans made that I'm aware of but I'm not on the match committee and I'm not close enough to it.''
Mr Kinley said there was no room on the calendar to extend the club season by a week should the fixtures be called off.
''We'll do everything we can to play the games and if the weather does turn really bad then we'll talk about it then.''
