Large-scale ocean atmosphere interactions play an important role in determining the interannual and multi-decadal variability of tropical cyclones in the Australian region. These large-scale environmental factors include sea surface temperature (SST), deep tropospheric vertical wind shear, and low-level relative vorticity. A principal component analysis of the SST data reveals two main modes of Pacific Ocean variability that match closely with basin-wide patterns of correlation between SST and TC frequency. Previously established statistical relationships between local and non-local SST, and Australian TC characteristics, are revisited using a 36-year data set from 1970/71-2005-06. Large, relatively stable correlations are found between seasonal TC number and Niņo 3.4/ Niņo 4 SST, with August-October Niņo 4 SST producing the highest correlation (-0.73). In contrast, much weaker correlations (+0.37) are found in the SST region to the north of Australia where many TCs develop. These correlations reduce to almost zero after the effect of Niņo 3.4 SST is removed by partial correlation analysis. When only data up to 1985 are considered, the positive correlation between local SST and TC frequency is substantially higher (+0.88). The annual frequency of TCs is strongly influenced by low-level relative vorticity and deep tropospheric wind shear in the Australian region that are modified by the slowly-varying tropical Pacific SSTs. Finally, while the overall trend in TC frequency is downward for 1970-2005, a pronounced upward trend is evident between 1986-2005, emphasizing caution when selecting start and end points of time series for analyzing relationships between predictors and TC activity.