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

Yes. Because of daylight saving time the gap moves between −6h and −5h.

When is the best time to call Sydney from Karachi?

Between 09:00 and 12:00 in Karachi, which is 14:00 to 17:00 in Sydney.

Karachi is 5h behind Sydney. This gap is not fixed: daylight saving time moves it between −6h and −5h during the year.

Karachi

Pakistan

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Sydney

Australia

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A 9-to-5 day in each city overlaps for 3h. That is 09:00–12:00 in Karachi, which is 14:00–17:00 in Sydney.

City	Time zone	UTC offset
Karachi · Pakistan	Asia/Karachi	UTC+5
Sydney · Australia	Australia/Sydney	UTC+10

What is the time difference between Karachi and Sydney?: Karachi is 5h behind Sydney. This gap is not fixed: daylight saving time moves it between −6h and −5h during the year.
Does the time difference between Karachi and Sydney change during the year?: Yes. Because of daylight saving time the gap moves between −6h and −5h.
When is the best time to call Sydney from Karachi?: Between 09:00 and 12:00 in Karachi, which is 14:00 to 17:00 in Sydney.