"It gets quite ferocious even in a smaller rain event like that. Any time the river's up you don't want to get in the water.
"It's a raging torrent basically. You'll have big boulders underneath the water moving along with that flow."
Moody said the man would have struggled to keep his head above water in those conditions.
A Wairarapa highway to a long weekend tourist hotspot has reopened but motorists were warned to watch for flooding on stretches of highway north of Wellington this morning.
Last night councils across the Wellington region closed a number of streets because of rising floodwaters.
The Hutt City Council closed Harcourt Werry Drive from Kennedy Good Bridge to Conolly St and asked people to stay off roads and avoid the rising Hutt River.
Block Rd in Lower Hutt was closed because of flooding but reopened just before 9am.
The NZTA said the Waihenga Bridge on SH53 was now under water, closing the road between Martinborough and Featherston. Detours are in place.
The unexpected road closure comes on the eve of the Martinborough Fair and long weekend holiday traffic.
The MetService said 88mm of rain was recorded at Kaitoke in the Akatarawa Valley yesterday as Lower Hutt bore the brunt of yesterday's deluge.