Chlorine gas fumes at the Bray Park Water Treatment Plant in Murwillumbah this morning have been contained and the situation resolved, with evacuated residents advised they are safe to return to their homes, Tweed Shire Council said.

Council staff alerted emergency services at 11.15am when a cloud of fumes was seen coming from a liquid chlorine tank which was in the process of being decommissioned.

Emergency services set up a 200-metre exclusion zone around the site as per their protocols. This zone was removed at around 2.30 this afternoon.

Emergency services assessed the situation and after taking gas readings, determined that the chlorine levels were well above safety threshold levels in the immediate vicinity of the tank.

Water was added to the tank in a controlled way to dilute the concentration in the tank, which was only about 12 per cent full. The gas generation was controlled and ultimately stopped.

Continued monitoring of concentration levels in the air during the process revealed that these levels soon returned to below the safe thresholds.

“The incident occurred when 700 litres of a proposed 1000 litres of sodium meta bi-sulphite was added to the tank,” Council’s Manager Water, Anthony Burnham, said.

“The chemical was being added to 3000 litres of remaining sodium hypochlorite in the 22,500-litre tank to de-chlorinate the remaining liquid to allow it to be safely discharged to sewer.”

This was part of the decommissioning of a disused section of the old plant on the site, in preparation for use by a community group establishing the Murwillumbah Men’s Shed.

“This chemical is routinely used on the site, however, in this case, a reaction occurred due to the higher concentration of chlorine in the tank,” Mr Burnham said.

The incident was in no way related the water supply or the operation of the new water treatment plant.

“Residents should not be concerned about their drinking water as the incident was in a part of the old plant which is no longer used,” Mr Burnham said.

“Council apologises to residents for any inconvenience caused by this incident and would like to thank all the emergency services for their quick and professional response.”