More than 40mm of rain has already fallen in the city.
Ealier today, the main road south of Oamaru was closed because of surface flooding.. State Highway 1 between Maheno and Oamaru was closed at 3.50pm.
MetService forecaster Paul Mallinson said Dunedin would experience the heaviest rainfall between 5pm and 8pm today as a rain band that battered Ashburton earlier this afternoon and is presently over Oamaru moved south along the east coast before turning inland.
As the rainband travelled it was breaking up, and Dunedin should not expect the same pummelling towns further north have received.
Three millimetres of rain fell on Oamaru between 3pm and 4pm, while 1.2mm fell on Dunedin, but between 5pm and 8pm rainfall could reach up to 5mm per hour in Dunedin.
That should be the peak of the rain which is then expected to gradually subside as the rainband heads inland, bringing scattered showers to inland Otago from tonight and tomorrow, and eventually dispersing over Fiordland, says Mr Mallinson.
Earlier, the Otago Regional Council (ORC) urged eastern Otago residents to be prepared as waters rise and the rainfall continued.
Staff rang farmers in areas of concerns to warn them of rising river levels.
ORC resource science manager Matt Hickey said farmers in the vicinty of the Kakanui and Shag Rivers on North Otago and the Silverstream in the Taieri were called this morning after a first alarm was activated by the rising rivers.
Further alarms were possible as rain continued to fall this afternoon, Mr Hickey said.
Farmers would be called again if a second alarm sounded, to warn them the threat of a flood was closer, but even a second alarm was activated well before rivers breached their banks.
The ORC was also watching the water level of the Leith Stream in Dunedin and the Tokomairiro River near Milton, which were also rising.
The ORC would continue to closely monitor the situation and issue public warnings as appropriate, he said.
SH1 and SH6 at the top of the North Island is open but Wairau Bridge on SH1 in Marlborough has been reduced to one lane. The road from Picton to SH1 and SH6 is open. This means motorists can travel between Picton and both Nelson and Blenheim respectively.
However, motorists will not be able to travel on SH1 from Picton to Christchurch due to extensive road closures caused by flooding.
SH1 is closed from Ward (south of Blenheim) to Saltwater Creek (south of the SH1/SH7 intersection), and also between Rakaia and Ashburton.
This means travel in and out of the Christchurch area has been significantly restricted.
Meanwhile police are expecting State Highway 1 between Hawksbury and Waikouaiti to flood at high tide this afternoon (2.30pm).
An alternative route to Waikouaiti via McGrath Rd has already been closed due to flooding.
Small creeks in the Omarama and Kurow area have burst their banks and surface flooding has been reported across North Otago.
Earlier report:
An already saturated Otago is on flood and slip watch as the MetService forecasts up to 100mm of rain in the east Otago hills as far south as the Rock and Pillar Ranges between 8am today and 3am tomorrow.
A heavy rain warning has been issued for Otago with forecasters warning the prolonged period of rain will continue to raise river levels and surface flooding is possible in some areas.
River flows in eastern Otago are slowly rising this morning, although most remain well below flood level, according to the Otago Regional Council's water information website.
Roads around the region remain clear as light rain falls in the city.
The storm moving slowly down the South Island, which is presently battering the Canterbury region, is expected to hit Otago later today.
A severe weather warning issued by MetService is advising people to look out for rapidly rising rivers and streams, as well as surface flooding and slips, during the period of heavy rain hitting eastern Otago today.
In the 15 hours from 6am to 9pm today, 60mm to 80mm of rain is likely about the Otago hills and ranges, with 30mm to 40mm nearer the coast, the service forecast.
Heavy rain is predicted to ease tonight, with patchy showers lasting until Monday.
A snow warning for the North Otago high country above about 700m was issued yesterday, warning of accumulations of 20cm to 30cm above about 900m to tonight.
Dunedin hydrologist Dave Stewart said the rainfall would visibly raise river levels in East Otago, but he doubted there was any risk of flooding.
Rivers in the area were running at their average flow for this time of year and because catchments were already saturated, any rainfall would run straight off into rivers.
Earlier, Metservice had warned people in Canterbury and Marlborough to watch out for rapidly rising rivers, while it lifted some of its severe weather warnings as the damaging storm moved south.
It said severe southeast gales in Buller and Westland should have eased by last night, but northerly gales over the north of the North Island would continue until this morning.
While heavy rain was expected to ease in Marlborough and the Richmond ranges last night, "further significant rainfalls" were forecast about Canterbury until this evening.
People in eastern Otago were warned to expect heavy rain today.
