The role of Southern Hemisphere (SH) stratospheric polar vortex variability for promoting Australian temperatures and rainfall extremes during austral spring-summer is explored. Anomalous weakening of the polar vortex during spring is shown to be associated with significantly increased chances of higher than normal maximum temperature (Tmax) and lower than normal rainfall during October-January across a large area of eastern Australia. The probability of occurrence of extreme events is also enhanced, with the chances of being in the top 20% for Tmax and bottom 20% for rainfall of October-January mean increasing by more than 4 times greater during vortex weakening years. This promotion of temperature and rainfall anomalies is shown to result from the associated swing to the low phase of the Southern Annular Mode that is induced by the downward coupling from the stratospheric vortex anomalies to the surface.  This vertical coupling that occurs on monthly to seasonal time scales implies enhanced predictability of temperature and rainfall extremes during vortex weakening years. This enhanced predictability is demonstrated with the Bureau of Meteorology ACCESS-S seasonal forecast system, which has a well resolved stratosphere and can capture the stratosphere to troposphere downward coupling.