Residents say systems are inadequate to deal with the water run-off from the dump. It warns that the run-off is reaching Hallards and Stringybark creeks, which run into the Ourimbah Creek, a major supply of fresh water for the Central Coast. Residents have also been given photographs that show a pipeline that appeared to be taking water from the dump site into a local creek. "We have lost faith in the EPA and we don't want to deal with them any more," association spokesman Dr Stephen Goodwin said. The EPA director of waste and resource recovery Steve Beaman told ABC Radio that they have had a breakdown in communications with the local community.

"Unfortunately we have lost the trust of the community and we need to rebuild that trust again," said Mr Beaman. The operator of the Mangrove Mountain landfill has been contacted for comment. A spokesman for Gosford City Council said the proposal to expand the landfill operations required a licence from the authority. Until that was issued, the council had no official role in relation to the site. He said the "council has previously raised concerns about the role of the NSW EPA as part of a parliamentary inquiry". "However, these concerns do not relate to the current licensing of the site."

A spokesman for the Minister Mr Speakman said the authority had been working closely with members of the Gosford community, including the Mountain Districts Association, to follow up concerns about potential effects on the Ourimbah Creek tributaries and drinking water supply. "The results from monitoring by water-quality experts from the Office of Environment and Heritage have not shown any evidence to suggest the landfill is having a negative impact on Ourimbah catchment water quality or stream health. This is also backed by the annual groundwater quality monitoring reports submitted to the EPA by the landfill operators, as well as catchment and drinking-water quality monitoring by Wyong Water," he said. "Given the Land and Environment Court determined in 2014 to allow the Mangrove Mountain landfill to remain and expand over the next decade, and that water sampling in the catchment has not shown any evidence of impact from the landfill, I do not consider an inquiry into the operation and regulation of this facility necessary." Stuart Khan, an associate professor in the School of Civil and Environmental Engineering at UNSW, said although there had been no clear unacceptable drinking water quality impacts, nor any meaningful increase to human health risks, indications of poor contaminant containment were cause for concern. "There are many potentially toxic substances, including perfluorinated compounds and brominated flame retardants, that have been shown to be leached from landfills, but are generally not closely monitored," Professor Khan said.

The authority has investigated the mega dump twice before – including after its own officers saw sediment-laden water being discharged into an adjacent creek. It is investigating claims sediment was washed into an unnamed creek in August. In 2015, the authority answered questions on notice from opposition spokeswoman on the environment, Penny Sharpe, revealing it had regularly issued cautions and letters to the operator for failure to comply with licence conditions. The authority said it issued a warning letter in 2011 for licence breaches, in 2009 for pollution of waters, and in 2008 for failing to apply "daily cover". Before that, it issued a warning letter in 2005 for "failing to sample all pollutants" and for late submission of a report. In 2004, an advisory letter and a pollution-reduction program were placed on the licence after a warning letter was issued in 2003 for failure to monitor all pollutants. But the authority said: "To date, the EPA has not been provided with or uncovered any evidence that the landfill is impacting on the drinking water supply."

The authority has confirmed there have been 26 reports of non-compliance on annual returns since 2006, but no fines have ever been issued to the landfill operator. The site has about 800,000 cubic tonnes of waste, with permission to expand to 1.3 million cubic metres of waste – which residents say is 25 times the original planning proposal. Residents have questioned data monitoring reports that showed arsenic and hexavalent chromium had been found in samples taken from ground water on the site. The authority spokeswoman said only three samples "exceeded the trigger levels of Australian drinking water guidelines for arsenic in 2010/2011. All results returned to below trigger levels in the following year." The samples were not from drinking water. She said sampling in the Ourimbah area, including downstream from the landfill, did not show any elevated levels of arsenic or chromium.