In winter, the usual position of the belt of westerly winds is close to Australia. Southern parts of the country typically get rainfall from cold fronts and troughs which pass over the southern reaches of Australia.
In a positive SAM phase, the belt of westerly winds contracts towards Antarctica. This results in weaker than normal westerly winds and higher pressures over southern Australia, restricting the passage of cold fronts inland. Generally, this means that there are fewer rain events in winter for southern Australia. However, in eastern Australia, the southward movement of the westerly winds means more easterly onshore flow is experienced. This wind is moist as it has just flowed from the Tasman and Coral seas and therefore typically brings more rainfall to the east.
In a negative SAM phase, the belt of westerly winds expands and is positioned more northwards towards the equator and Australia. This results in stronger than normal westerly winds, lower atmospheric pressure, more cold fronts and more storm systems over southern Australia. Typically this means that there are more rain events in winter for southern Australia. However, in eastern Australia, the northward displacement of the westerly winds means less moist onshore flow from the east, and thus decreases rainfall for eastern Australia.