This study examines decadal changes in the in-phase relationship between the Indian summer monsoon and following Australian summer monsoon using observational data from 1950-2005. The in-phase relationship is the tendency for a strong (weak) Indian summer monsoon to be followed by a strong (weak) Australian summer monsoon. It was found that the in-phase relationship was weak during the late 1950s and early 1960s, strengthened to a maximum in the early 1970s just before the 1976/77 climate shift, then declined until the late 1990s. This study shows that Pacific SST anomalies always have high persistence from boreal summer to austral summer to provide the needed memory to connect the Indian and following Australian summer monsoon. The correlation between the Pacific SST anomalies and the Indian summer monsoon is also always strong. It is the weakening and strengthening of the correlation between the Australian summer monsoon and the Pacific SST anomalies that contribute to the decadal variation of the in-phase monsoon relation. Related to a weakening monsoon-SST correlation, the lead/lag correlations between Pacific SST and the Indian and Australian summer monsoons became less biennial in recent decades.