Homes have been evacuated and roads closed after river levels reached a record high in Monmouth.
Water levels on the River Wye reached 7.15m on Tuesday morning.
Residents living in the areas at immediate risk have been evacuated and other properties nearby were provided with sand bags and support.
Natural Resources Wales warned the flooding could be a "danger to life."
The river has overflowed and the A40 between Monmouth and Ross on Wye is flooded as a result. The road is closed from the Monmouth traffic lights and the Wye Bridge is also closed.
Gwent Police are advising people to only travel if it is essential.
The grounds and sports field around Monmouth Comprehensive School are flooded and Monmouth School's pre-Prep and nursery building has also been hit by floodwater.
Monmouth Leisure Centre has also been affected by the floods and is closed due to the access being completely submerged.
Matt Jones from South Wales Fire Service said that after a "relatively quiet night" water levels started to rise this morning on the River Wye.
Since then, he said they have "dealt with a number of incidents" and rescued one 65-year-old lady who was stuck in her car.
Mr Jones said water levels are "staying stagnant at the moment" but with more rain expected, they "do have concerns."
If levels do rise again, he said they are prepared to "deal with any incidents."
Meanwhile residents in Monmouth town, Osbaston, Wyesham, Manson, Overmonnow, The Kymin and Kings Fee are being asked to reduce their water consumption due to flooding and a power cut at the water treatment works in Mayhill.
Welsh Water says it has "limited storage of drinking water" until it is able to access and restart the treatment works.
A spokesperson said bottled water is in the area should supply be lost, but warned due to flooded roads it is proving difficult to bring lorries into the town itself.
The spokesperson added: “Customers in vulnerable circumstances – such as the elderly, infirm or with young children – will be prioritised for bottled water, if needed, and such customers should contact us on 0800 05200130 or via social media (Twitter: @dwrcymru or on Facebook) to request bottled water if they’re not already registered on our priority services register.
“We would like to assure customers that we are working hard to maintain water supplies to the area for as long as possible and, in the event that the supply is interrupted, will we restore is as quickly as possible."
Many roads across south Wales remain impassable in the aftermath of Storm Dennis.
A full list of road closures can be found on the Welsh Government's Traffic Wales website.
Rail lines also continue to be disrupted with many Transport for Wales services affected.
Passengers are being advised to check their journeys ahead of time and allow more time for travel.