The "not-for-profit" company which owns Dwr Cymru Welsh Water has made pre-tax profits of £187m, down five per cent on the previous year.
Glas Cymru was given approval to take over Welsh Water in 2001
Glas Cymru has already announced plans to put up bills by 16% next year as part of a 23% increase over five years.
Consumer watchdog group WaterVoice Wales has already voiced concern about the rise.
But the company defended the increase, which it said was needed to meet water standards under European legislation.
Announcing its results on Thursday, Glas Cymru said it had invested £1bn over the past four years.
It has also put forward investment plans up to 2010, including cutting flooding risks and improving water quality. About half of this will be paid for by customers.
By 2010 Welsh Water's average household bill will be £353 at today's prices, up £67, plus inflation on next year's bill.
Nigel Annett, executive director of Glas Cymru, said the company did not want to increase bills, but it was obliged to raise water quality.
He told BBC Radio Wales that profits were lower this year because the company had promised to cut bills if it hit its targets. That resulted in £9 bill rebates in each of the past two years.
Mr Annett said water bills had not risen over the past 15 years, and were actually lower than four years ago.
"Because of the investment that we have to carry out to meet new standards, largely in line with European legislation, then bills will have to rise over the next five years," he told Good Morning Wales.
"The last thing we want as Glas Cymru is to see bills for our customers going up," he said.
But Clive Sterl of WaterVoice Wales said the proposed bill increase was more than four times as big as any rise since Welsh Water was privatised, and customers would not be able to absorb it.
Mr Sterl said his group wanted to ensure that Glas Cymru was "leaning" on its contractors to cut costs.
"What Glas Cymru does is farm out all its work to other water companies on a sub-contract basis.
"These companies are in business to make a profit," said Mr Sterl.
"So the notion that somehow there is no profit and customers can't lose is a little bit loose."