Prime Minister Jacinda Ardern travelled to the region on Sunday and Monday, assessing damage and comforting locals.
Nelson received more than three times its average August rainfall in four-and-a-half days last week, with 274mm falling at the airport.
In hills just six kilometres south, one rain gauge measured 1013mm, which produced huge run-offs and landslips.
Some residents have lost their homes, while others remain unsure whether they will have anything to go back to.
Of the 284 properties impacted by the storms to be assessed so far, Nelson Tasman Civil Defence report nine as uninhabitable and 11 as seriously unstable.
Nelson Mayor Rachel Reese said there were another 286 homes still to be assessed.
"We're working our way through it ... it needs to dry out some more to really be able to refine those assessments," she told Radio NZ.
"There are going to be people in our city who are not going to be able to return to their homes.
"It's incredibly difficult."
Many residents have been unable to even access their homes due to widespread road closures.
Insurance Council chief executive Tim Grafton told Radio NZ he envisaged thousands of post-flood claims from Nelson alone.
"It's going to be big," he said.
State Highway 6 is closed along the Nelson foreshore and between Nelson and Blenheim, with major works required, while State Highway 63, another arterial route through the Wairau Valley, is also closed.
MetService spokesman Dan Corrigan said more rain could be on the way.
Nelson is on the fringe of a low-pressure system due to cross the area on Monday afternoon, with forecasters watching another front coming from the south and due to hit on Thursday.
"It's been absolutely extraordinary compared to what they're used to," Corrigan said.
"There's a chance they might not see very much rain at all today ... then at least there's a couple of dry days until Thursday."
Civil Defence have set up sites for locals to go and dump mud and soil as the big clean-up begins.
The local Maori tribe, Te Tauihu iwi, have placed a rahui - or ban - on entering the water across the South Island's entire northern coastline in response to the events.
