"Sydney Water and the Environment Protection Authority [the regulator] are both failing to meet the community's legitimate expectations, and we call on the Baird government to tell the public how it intends to rectify this intolerable situation," she said. A spokeswoman for the EPA said the North Head and Malabar plants complied with toxicity limits within Sydney Water's licence. Experts have said the sudden spike in mercury being pumped out to sea at North Head - the highest level in five years in Sydney - is puzzling and also concerning, particularly after the release of a report in February warning that sharks being caught off the NSW coast had high levels of mercury which could have implications for public health. The report, by scientists from the National Marine Science Centre, the Marine Ecology Research Centre and the School of Health and Human Sciences at the Southern Cross University and the NSW Fisheries department, was published in the Marine Pollution Bulletin journal. It warned that consumption of some species of shark commonly used in fish and chips should be limited. It also follows an important 2008 report by Associate Professor Stephen Corbett, the director of the Centre for Population Health, published in the Medical Journal of Australia, showing that children in Sydney who were eating almost daily serves of fish had mercury levels up to seven times the maximum safe level and were being treated for developmental delays or neurological problems.

Chemistry professor Ian Rae, currently an honorary professorial fellow in the Faculty of Arts at the University of Melbourne, said the spike in mercury was "puzzling". He said it may be from trade waste. Water quality expert, Dr Ian Wright, from the University of Western Sydney, said the amount of mercury reflected the "sort of trade waste that Sydney Water is willing to accept". Sydney Water has sewage plants at North Head and Malabar where it discharges sewage to the ocean after only what is called "primary treatment" - scraping the solids out of the waste that fall to the bottom of the tank. A spokesman for Sydney Water said while it does not produce the mercury, it does accept trade wastewater from businesses and industry that can include traces of mercury and other metals. He said the amount of mercury discharged yearly varies, as samples are taken once a quarter and used as an indication of yearly discharge. He said some of the reasons for the mercury spike might be accumulated silt and rubbish washing into the treatment plants during heavy rainfall, waste from dental facilities, personal dental fillings, and possibly illegal sewer connections.

But he said the variation in amounts of mercury emitted each year each "cannot be attributed to deterioration or improvements within our systems, nor can it be used to suggest more or less mercury is being disposed of through our network". He said the levels discharged were well within the licence limits set by the EPA for both North Head, which allows 60 kilograms, and Malabar which is allowed to dump more than 100 kilograms. The spokesman also said that Sydney Water undertakes ocean sediment monitoring to see if there is any impact on marine ecosystem health at the deep ocean outfalls. "The program has been in place for over 10 years with data showing no measurable impact on marine ecosystems." Associate Professor at the School of Civil & Environmental Engineering UNSW Stuart Khan said without a more focused study it is not immediately possible to know how much of the mercury load found in the sharks may have come from sewage discharges.

But he did say the high rate of emission of mercury to the ocean from sewage ocean outfalls is a "largely unseen and dormant problem". Professor Khan said improving the treatment process before the sewage is pumped out to sea would be helpful. "The real solution though would be to apply more advanced treatment processes, which are commonly employed at sewage treatment plants around the world. In the US, the EPA has banned the discharge of primary sewage to the ocean," he said.