@BNEA320 - I use the trains at Sydney airport all of the time. It's a great service and their are train station underneath both domestic and international terminals.
From memory, the return train fair from Domestic to Central station in the city costs $16. A taxi for the same would cost maybe $80 and take a lot longer.
Just walk out of the Domestic exit gate, down the escalator to the right and then take another escalator downstairs to the railway station. It's a really well integrated into the airport, quick, efficient and clean.