The Environment Agency is warning that 75% of people whose homes are at risk of flooding in Wales have failed to take action to protect their property.
Recent floods have hit homes in the Merthyr and Swansea areas
It has also warned that an increasing number of people are at risk as a result of climate change.
It described some people as being "in denial," with only 20% at risk signed up to its direct flood warning service.
Properties in the north Wales' coastal areas are offered the phone, email or text service for the first time.
Environment Agency Wales Director Chris Mills said: "A lot of people actually living in the flood plain are unaware or indeed in denial of the risks they are facing.
"It's surprising that so many people take no steps whatsoever to safeguard their home against the increasing risk of flooding - even though they all lock their doors to protect their homes and possessions against burglary."
The agency calculates that flooded homeowners stand to lose on average more than £28,000, compared to a loss of £900 if they are burgled.
Of the 170,000 homes and businesses at risk from flooding in Wales, the EA said it could currently provide a direct flood warning service to more than 90,000.
But only 20% of these have registered with the service, despite being contacted through flood awareness campaigns.
Richard White, a councillor in the Gwent levels area which lies below the high tide mark but is protected by a sea wall, said the danger from flooding is on the back of people's minds.
He said: "There is always the possibility that you can get freak on-shore winds combined with high spring tides, which could cause an overspill.
"With the climate change warnings that we're receiving all the time, there's a possibility that sea levels will rise and the sea wall in future will have to be raised even higher."
Mary Dhonau is co-ordinator of the National Flood Forum, set up after the floods of 2000, said a number of practical measures, such as covering up air bricks, could cut the damage by flooding.
She said: "Anybody at risk that doesn't take moves to help themselves, especially with climate change is really deluding themselves and they're making a very big mistake."
Jeff Jones, a local government consultant, said: "Much of localised flooding is not caused by houses being built on flood plains, it's a drainage issue.
"The drains are not able to cope with a huge increase in water or were badly planned in the past."
More than 4,000 homes and businesses in areas of north Wales will be offered the environment agency's flood warning service.
The EA is running road shows across Wales to raise awareness of the risks of flooding, which continue at Kinmel Bay community resource centre on Wednesday, 1000 -1800 GMT.
Homeowners can also find out if their home is at risk by logging on to www.environmentagency. gov.uk/floodline or calling floodline on 0845 988 1188.