The average domestic water and sewerage bill is currently already higher in Northern Ireland at around £408 a year or £1.12 per day.

This compares to an average bill of £379 in England and Wales.

The comparative NIW bill does not include the costs of domestic billing and domestic debt recovery costs so charges would mean bills would soar above those in England and Wales.

The company said it has fewer financial incentives to outperform and faces more uncertainty over long-term investment because it is owned by the Government and yet is operated as a quango.

"Working as both a government company and a non-departmental public body creates organisational strains on operating both of the delivery models," a statement read.

"Compared to our comparator group in England and Wales, we have fewer financial incentives to outperform; face more uncertainty over long-term investment planning, resulting in a suboptimum capital investment delivery model; and have less commercial freedoms to restructure and make decisions."

The comments followed the company's annual report. The running of NIW is the responsibility of the company's board but the Department for Regional Development monitors its performance and outlook.

NIW's annual report said customer service had improved and the company had invested more than £1.3 billion since its creation in 2007. A further £147 million is scheduled to be invested by March 2013.

The figures released showed the company made a profit of more than £133m this year, up from £112m in 2011.

But Ronan Larkin, NI Water's Director of Finance and Regulation stressed that any profit generated is handed back each year for public expenditure.

Mr Larkin said 50% of available investment funds will go into maintaining the systems and the rest into upgrading services.

He added that Northern Ireland Water had been increasing its efficiency "within the model that we are given to operate."

"Any questions as to how that would be taken forward we would look to the assembly here to take that model forward and work with them on that," he added.

Mr Larkin also said following an assembly review into the recent flooding problems in Belfast and other areas, any additional funding would be used to improve services further.

"We would seek to work with the assembly executive to make sure those monies are invested for improved services and for any out of sewer flooding that might be alleviated going forward."

Northern Ireland is one of the few regions in Europe without domestic water or sewerage charges. More than 65% of NIW's income, which is around £269 million, is in the form of government subsidy.

The efficiency gap between Northern Ireland and England and Wales has been reduced from 49% then to 34% in 2010/11. NIW hopes to report further reductions in this efficiency gap following operating cost savings worth £12 million in 2011/12.

Northern Ireland's antiquated system of sewerage piping is currently being overhauled and new waste treatment works put in place while efforts continue to reduce leakage of drinking water following the crisis two years ago when thousands were cut off following burst pipes caused by winter weather.

NIW chairman Sean Hogan said: "Our focus over this period remains on prioritising customers through investing in our infrastructure, improving how we deliver our service and operating more efficiently."