We analyze P-wave amplitude reduction for Tonga earthquakes observed at the SKIPPY broadband seismograph network in Australia. The negative amplitude anomaly diminishes with a focal depth, suggesting that the anomaly is due to defocusing caused by the fast Tonga slab. The SKIPPY network covers the distance range where the effect of the Tonga slab is most pronounced. We calculate theoretical amplitudes using simplified slab models based on previous delay time tomography, and show that the theoretical amplitude anomaly reproduces the observations reasonably well. A 100-km thick slab provides a better fit to the data than a 50-km thick slab. A thin slab penetrating deep into the lower mantle is inconsistent with the pattern of amplitude variation with distance. ¿ 1999 American Geophysical Union