Fire and Emergency New Zealand (Fenz) Nelson-Marlborough assistant commander Chris Best said the water had begun to recede after two hours of heavy rain on Monday morning.
"The result is that we've got many blocked drains around the Nelson area and it has caused a large amount of flooding.
"It's subsided now, so that's a relief, but we've had multiple calls backed up and we've had appliances come in from as far as Stoke and Hira to help us out, along with the four appliances from Nelson."
While houses had been affected, Best said it was unlikely to be a particularly damaging deluge.
"We haven't had much water enter the houses - into some garages, but nothing major," he said.
"As I say, the drains have taken effect again and the water has cleared. So I can't imagine it being a major damaging event."
"But we're always ready and that's the main thing," Best said. "And in this case we've got plenty of resources that came in to help us out, and with the calls backed up, it took about 30 minutes to clear them.
"It's just a timely reminder we weren't under a weather warning, but what we need is for people to ensure that their drains are clear, that their gutters are clear with it being autumn and the leaves are falling and even the drains that are on the road if they see that they might be a cause for concern to unblock them and ensure that water can escape."
"Some of the flood waters are quite deep, so please treat all flood waters as potentially contaminated," Nelson Tasman Civil Defence said on social media earlier.
MetService did not have any watches or warnings in place for the Nelson region, but its rain radar showed a band of 'moderate' rainfall crossing the upper South Island on Monday morning.
Fenz Southern Communications Centre shift manager Lyn Crossan said earlier there had been more than 20 incidents reported since 10.15am, most of them in the area between Washington St in the north and Bishopdale in the south.
"We are prioritising the most urgent incidents first, where there is danger to people's safety," she said. "However, the majority of incidents relate to floodwaters entering property. We are strongly advising people to avoid travelling to or through that area this morning."
One local said cows were being washed away by floodwaters.
"[My husband] went to check on our neighbours then the drains in the road to make sure they weren't blocked," Brittany Van de Waardt told RNZ.
"That's when he saw the two cows in the river. The river is very high and rough at the moment, one of the cows looked to be deceased and was getting tumbled down the river. The other was alive but struggling.
"We haven't seen any more, but I wouldn't be surprised if there was other livestock. He just happened to look across to the river at the right time. Just sad for the animals and farmer."
"By [the] time we went back out, they were far down the river."
It was also reported manhole covers were being pushed out of their holes.
Nelson City Council group manager infrastructure Alec Louverdis said earlier the council was "actively monitoring the weather conditions and responding to surface flooding in multiple locations across the region".
"Rutherford Street will be closed shortly while urgent repairs are carried out on a manhole. Contractors are currently on site managing the situation."
He said the eBus service was temporarily suspended, and routes were being assessed for safety and accessibility.
"Residents are advised to delay travel if possible. If you must drive, please reduce your speed and avoid flooded roads. Please stay clear of floodwaters, which may be contaminated and pose health risks.
"The weather front is now moving away from the region, but council will continue to monitor the situation and respond as needed."
Further south, NZ Transport Agency Waka Kotahi said State Highway 6 from the SH6/SH69 Inangahua Jnc to the SH6/SH67 Lower Buller Gorge intersections would be closed "due to multiple slips and flooding".
"Delay your journey or... use an alternate route," it said on X (formerly Twitter).
