We use data from the GEOSCOPE global seismograph network to determine source parameters for the 23 May 1989 Macquarie Ridge earthquakes. By moment tensor inversion of long-period surface waves we obtain a seismic moment of 1.8--2.4¿1021 N m, which makes this the largest seismic event to have occurred worldwide since the 1977 Sumba earthquake. The focal mechanism shows pure right-lateral strike-slip motion on a plane parallel with Macquarie Ridge and the Pacific-Australian plate boundary. The mechanism is consistent with the strike-slip motion predicted for this area from relative plate motion models. We invert broad band teleseismic P and SH waveforms to determine source complexity, using the results from the long-period analysis as a priori soft constraints. The broad band waveforms show that the release of seismic energy was concentrated in the first 30 s of seismic rupture, but fault slip continued for more than 60 s. Lack of directivity in the long-period and broad band waveforms indicates that the rupture propagated bilaterally or vertically from the nucleation point. ¿ American Geophysical Union 1990