Closures and up-to-date information are posted on http://nelsontasmancivildefence.co.nz/
Authorities are warning people to stay away from rivers and affected areas.
St John Ambulance in the Richmond area had moved its staff and vehicles because of flooding and was operating from stations in Nelson, Motueka and from a motel in nearby Stoke.
It said it was too early to know what damage the Richmond station had suffered.
Chris Haines, St John regional operations manager South Island, said that with the state of emergency declared, St John was on stand by and working with other emergency services and Civil Defence as required.
Its crews were preparing for high tide.
Earlier, a Fire Service spokesman said firefighters had been called to free a trapped person after two cars collided in bad weather near the Nelson Lakes town of Murchison.
He said the person was taken to hospital in a serious condition.
Most other callouts were over surface flooding in Nelson, he said.
Nelson has been hit hard by heavy rain, with one motorist narrowly avoiding disaster when dislodged debris struck threatened to push his car into the ocean.
Lawyer Rick Farr said he watched as a rock "half the size of a car" slammed into the side of the four wheel drive just metres ahead of him in a queue of traffic.
"There was just an explosion from the side of the hill and suddenly we can see very large boulders heading towards us.
"It was a split second and stuff came right at this guy's truck and slammed into the side of him and then stopped. It's remarkable he wasn't pushed over... He's the luckiest man alive."
The driver appeared unhurt as he pulled his car over to the side of the road to check for damage, Mr Farr said.
He said Rocks Rd was prone to erosion and was dangerous in wet weather.
"They need to do something about it or someone's going to be killed one day."
Meanwhile, two tourists have had a lucky escape after being rescued from trees they climbed to get away from flood waters in Nelson.
The pair from Germany and Canada got into difficulty after going for a walk at lunchtime to check out Nelson's flooded Maitai River.
Sergeant Steve Savage of Nelson Police said that was when one of the men made a very serious mistake.
"One had foolhardily entered the water to see how swift the water was, and got into trouble. The other male had gone to help him and they both ended up stuck in a tree,'' he says.
Sergeant Savage said the pair were up the trees for just over an hour before being collected by search and rescue volunteers.
Weatherwatch.co.nz said the torrential rain hitting the town was set to spread across the North Island on Thursday.
Head weather analyst Philip Duncan said the system could create explosive local downpours and lead to flash flooding.
"Flash flooding is when a usually small stream, creek or drain turns into a raging torrent within minutes and can be deadly," he said.
"This is expected in isolated pockets right up the western and northern coastlines of both islands, with the focus shifting more to the North Island today".
MetService said significant rain was expected to hit many areas across New Zealand in the coming days as an active front moves eastward across the Tasman Sea.
Most of the country was forecast to receive showers today, with the heaviest rain expected in west and north of both islands.
Large amounts of rain are likely today and early tomorrow for Westland and Buller, Marlborough, and for Nelson where as much as 400mm of rain may fall about the northwest ranges.
Nelson City Council earlier said it was laying precautionary sandbags in areas that could be affected if the Maitai River continues to rise.
The council said several roads have been closed as a result of surface flooding due to the level of the Maitai River including the Maitai River Road, the Manuka Street Ford, Avon Terrace, Hanby Place and Nile St near the river.
The Maitai Walkway is also closed to pedestrians and cyclists.
- Herald Online staff
