Girling Road is currently flooded as torrents of water spreads across the street.
Those living in Highfields, Quebec Road, Sandy Lane, Theatre Street, Becclesgate, Moorgate, Etling View, Northgate, Fox Road and Oak Road are among those who have lodged problems with Anglian Water.
The Red Lion Pub has had to close temporarily while the issue is ongoing, as has the Orion Cinema.
People living as far as Swanton Morley are also experience problems with the water supply.
A spokesman for Anglian Water said: “We’re really sorry but some customers in Dereham may have either very low water pressure or no water at all.
“This is being caused by a problem with one of our pumps that push water around the network.
“Based on the latest information available, we’re aiming to have everything back to normal by 9.30pm.”
Temperatures are due to peak in Dereham at 28C on Sunday afternoon.
One Dereham local, who wishes to remain anonymous, said: “We are currently affected by this and have no cold water.
“I contacted anglian Water who advised they will not be supplying bottled water, even though by the time they are anticipating the problem is fixed it will have been off for more than eight hours.
“This is absolutely unacceptable on the hottest day of the year when guidance is to keep hydrated.”