We have learned that whenever the Torah deals with relative times, the time expressed in the Torah is that of Eretz Yisrael. For example, the molad announced is Yerushalaim Solar Time (not even Israeli clock time which is 20 minutes later). Similarly, there is a commentary when Moshe Rabbeinu said "about midnight" he was speaking of Midnight Yerushalaim time rather than midnight in Egypt.
Kehillas Yitzchak says:
Hashem's 'clock' is set to Jerusalem. The plague of the first-born
took place at exactly midnight, Jerusalem time. But because Egypt is
west of Jerusalem, midnight there occurs later. When Moshe said the
plague would be at 'approximately midnight,' he was referring to
local, Egyptian time.
(Kehillas Yitzchak; thanks to Rabbi Sholem Fishbane)
See When is “vesain tal umotor” said in Australia? which shows how seasonal changes follow Eretz Yisrael.
Similarly, since leap year is based on Aviv in Eretz Yisrael, this shows that Australia would also have leap year based on the same calender and the circumstances in Eretz Yisrael.