Western Outfall Main Sewer is a heritage-listed former sewage farm outfall sewer and now ocean outfall sewer near Valda Avenue, Arncliffe, Bayside Council, New South Wales, Australia. It was designed by the New South Wales Department of Public Works, who built the sewer from 1895 to 1898. It is also known as SWSOOS No. 1, Western Main Carrier and Western Main Outfall Sewer. The property is owned by Sydney Water. It was added to the New South Wales State Heritage Register on 15 November 2002.