Does the time difference between Mumbai and Sydney change during the year?

Yes. Because of daylight saving time the gap moves between −5h 30m and −4h 30m.

When is the best time to call Sydney from Mumbai?

Between 09:00 and 12:30 in Mumbai, which is 13:30 to 17:00 in Sydney.

Mumbai is 4h 30m behind Sydney. This gap is not fixed: daylight saving time moves it between −5h 30m and −4h 30m during the year.

Mumbai

India

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Sydney

Australia

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A 9-to-5 day in each city overlaps for 3h 30m. That is 09:00–12:30 in Mumbai, which is 13:30–17:00 in Sydney.

City	Time zone	UTC offset
Mumbai · India	Asia/Kolkata	UTC+5:30
Sydney · Australia	Australia/Sydney	UTC+10

What is the time difference between Mumbai and Sydney?: Mumbai is 4h 30m behind Sydney. This gap is not fixed: daylight saving time moves it between −5h 30m and −4h 30m during the year.
Does the time difference between Mumbai and Sydney change during the year?: Yes. Because of daylight saving time the gap moves between −5h 30m and −4h 30m.
When is the best time to call Sydney from Mumbai?: Between 09:00 and 12:30 in Mumbai, which is 13:30 to 17:00 in Sydney.