Iron concentrations in a rivulet that feeds the Woronora Reservoir have doubled since the expansion of the Metropolitan Colliery, to levels which exceed Australian Drinking Water Guidelines by 30 per cent, a study has found.

Until an independent investigation into its impact on Sydney's water supply has been done, a member of the mine's community consultative committee wants the state government to halt operations.

Iron contamination near the Metropolitan coal mine. Source: Peter Turner

''The mine expansion approval conditions clearly state that there should be no more than a negligible reduction in water quality and quantity,'' said Dr Peter Turner, the National Parks Association-nominated committee member who reviewed the mine's 2012 annual report.

''These requirements are there to protect water supply and must be taken seriously.''