A TOP building firm has been banned from bidding for Housing Authority contracts worth $4 billion after it mistakenly piped toilet water into residents' taps. Shui On, the winner of five awards from the authority, has been barred from competing for seven large housing projects during its three-month suspension. The company has apologised and admitted responsibility for the work, which led to contaminated water being piped to 1,122 homes in Chung On Estate and Kam Fung Court this month. Occupants are fighting a legal battle over the company's offer of compensation. The consultant architect and plumber have been blacklisted for six months. The authority's legal adviser, Simon Lee Pak-ho, said the action was severe but necessary, 'taking into account the seriousness of the incident'. Shui On builds more than 10 per cent of authority flats and expects to lose at least $700 million from projects it believed itself certain to secure. Managing director Lawrence Choi Yuk-keung said Shui On would 'try to cope' with losing the source of more than half its work: 'We've done work for the authority for the last 10 or 15 years and have an excellent record. I don't think we deserve this.' None of the company's current 10 authority projects is affected by the move. Liberal Party legislator Edward Ho Sing-tin, managing director of the blacklisted architectural firm Wong Tung and Partners Ltd, said the firm had not admitted blame and would consider acting against an 'unfair' decision. A Housing Department investigation revealed that the contractor had rearranged underground water pipes at three T-junctions without sending drawings to its consultant architect. It then failed to check the pipes before connecting them.