Fiji started fasting on Saturday and Australia didnot see the moon as expected on Sunday so they followed the sunnah and started fasting on Tuesday. Some other countries started fasting on Monday. Some countries celebrate EidalFitr on different days also.
The start and end of Ramadaan is based on the sighting of the moon. Nabi (Sallallaahu Alayhi Wasallam) said, “Fast when you see it (the moon ) .. .. Based on the Hadith people of the country look for the moon and e.g. if in Fiji they sight the moon they fast and if in Australia it is overcast and cannot see the moon they need to complete the 30th day of Shabaan and then fast, thus, the differences in days in the different countries.
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai