Meteorologist Peter Little said the rain had been torrential in Milford area this morning, and 250mm of rain has fallen since midday yesterday in the ranges.
"A station at milford Airport recorded 44mm in one hour at 9am this morning -- that's a lot of rain, that's torrential and would no doubt cause surface flooding," he said.
About 100mm of rain has fallen at Mt Cook, while Hokitika had received 60mm of rain and Westport 54mm, Mr Little said.
WeatherWatch said the wet weather today was the beginning of nine days of rain for the South Island.
Mr Little said the very heavy rain would be moving northwards today and into Thursday, but at a very slow pace.
"It is unlike the fronts we've had this year ... it is very, very slow moving."
Up to half a metre of rain was forecast to fall across the South Island today and tomorrow, Mr Little said.
For the Otago and Canterbury headwaters away from the main divide, and the Buller and northwest Nelson ranges, between 150mm and 250mm was expected today.
MetService advised that streams and rivers would rise rapidly, which would affect areas further downstream, including in Canterbury and Otago.
Surface flooding and slips were likely.
For the Tararua range, Taranaki and the central North Island high country, 120mm to 150mm was possible in a 15 to 18 hour period during Thursday and early Friday.
Winds would also cause problems around the Canterbury High Country.
MetService has warned winds could gust up to 130km/h, which was likely to bring down trees, powerlines and make driving conditions hazardous for high-sided vehicles.
Meanwhile, Nelson City Council is bracing for the heavy rain to make its way north.
Residents in Nelson have been urged to clear leaves and other rubbish from any gutters and stormwater grills near their property.
The council warned that if sumps were blocked by leaves or other debris, rain water can quickly build up and add to the risk of flood damage.
MetService has warned the heavy rain and severe gales from the weather bomb would move north to Bay of Plenty on Friday.
Auckland and Northland could also be affected by the rain and wind.
The heavy rain and strong winds would continue for parts of the South Island over the weekend and into next week.
On Saturday another front is expected to move northeast over the lower South Island and MetService said heavy rain in Westland and Fiordland was likely.
On Sunday moist northwesterlies affect the country and would likely bring heavy rain in Fiordland, Westland and Buller.
At the beginning of next week an intense trough is expected to approach the country from the west heavy rain in Fiordland, Westland and Buller was again likely. MetService was moderately confidence of severe northwesterly gales in Canterbury,Marlborough, Wellington and Wairarapa on Monday.
