The Water Supplies Department said it would try to improve meter maintenance after being chided by the Ombudsman over defects that led some consumers to unexpectedly receive bills for tens of thousands of dollars. Ombudsman Alan Lai Nin said it was also not ideal when it could take the department years to identify a defective meter as there were so many. His comments followed an investigation into the department's procedures and the issuing and adjusting of bills, following 7,300 complaints about defective meters and a lack of explanation on bill calculation. In one case, an owner's corporation received a demand for HK$25,000 as the department said it had been undercharged because of a faulty meter. 'The department randomly inspects about 500 meters every month and results show about 5 per cent of them are not accurate,' Lai said. 'Five per cent may sound like a small amount, but when you realise it means about 140,000 of the 2.8 million meters across the city, it is not small any more.' The investigation found inadequate training and supervision of meter readers and insufficient system checks to identify defects caused delays in fixing problems and bills for shocking amounts. The owner's corporation cited in the report had received eight 'zero' bills for a meter serving the common area of the building between early 2006 and late 2008. It was not told until May 2009 that the meter was inaccurate and needed replacing and it took a further four months for the new one to be installed. The corporation then got the HK$25,000 bill for the period between July 2005 and September 2009. Lai said the 'long period needed to fix the defective meter' led to a complicated problem. In a statement last night, the department agreed with the Ombudsman's recommendations and said it would try to enhance staff training and improve its computer system.