A Wingecarribee Councillor says claims that high concentrations of heavy metals are being released from the Medway Colliery near Berrima are a fallacy.

The claims were made in submissions to the Planning Assessment Commission, before the state government body gave approval for Boral to double coal production at the historic mine.

While Boral claims any contaminants in  water releases are within stipulated limits, Peter Martin from the Coal Action Group says he has major concerns, given the flows eventually end up in Sydney's main water storage, Warragamba Dam.

Cr Graham Mc Laughlin, who works for Boral, says water from the river is used in local ponds, and has had no effect on fish life, so he  finds it impossible to believe flows are  contaminated.

A number of homeowners who draw water from the river say they have no concerns about the water quality, and are worried what will happen if the supply is shut off.