The Consumer Council for Water, which helps resolve customer complaints, said the bill rises “may be less than what water companies wanted but they are still more than what many people can afford”.
It said customers would be “hit particularly hard from April”.
Mike Keil, its chief executive, said at least two in five households in England and Wales “will struggle to afford” these increases.
Social tariffs which give a discount on bills are available for people who need help.
But who actually gets financial help is “a roulette”, he told the BBC’s Today programme, because each company sets its own criteria for who qualifies.
Citizens Advice warned that water bill increases could push people further into debt unless struggling households get more support.
Angela Newey, chief executive of Citizens Advice in north and west Kent, said the rise “may not look very much”, but added it would “squeeze people’s budgets and then it will have a knock on effect on their other bills”.
“People are really struggling with their water bills. It doesn’t tend to be what they come in for, but when we go through their finances we’ll see that water is generally an issue, and what we do find is that it’s very often the first bill they won’t pay,” she said.
Help is at hand for people who are struggling, including social tariffs offered by water companies, but they are not always aware of it, she added.
“If people contact their water companies there’s actually quite a lot of help that they can give, particularly if they’re on a low income, or if they have a disability,” Ms Newey said.