A new expression with superior consistency in the calculation of magnitude from earthquake coda duration has been developed. The need to take into consideration the site, the seismograph characteristics and the source has been investigated. The expres sion is valid for distances up to 1000 km in southeast Australia except where the source is located on some other major geological foundation, for example the Australian shield or the Tasman Sea. The expression takes the form: MD = PI + p2(log(SDDaP3 + P4Re-P,R MD is duration magnitude, and the parameters in the above expression are determined by regression based on local magnitude ML. Duration, D, is in seconds, R is the hypocentral distance in kilometres, and Pn are parameters. Parameter PI zeroes the function to give values similar to ML and accounts for the definition of duration; P2 and P3 give the shape of the variation