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

Yes. Because of daylight saving time the gap moves between −4h and −3h.

When is the best time to call Sydney from Bangkok?

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

Bangkok is 3h behind Sydney. This gap is not fixed: daylight saving time moves it between −4h and −3h during the year.

Bangkok

Thailand

--:--:--

Sydney

Australia

--:--:--

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

City	Time zone	UTC offset
Bangkok · Thailand	Asia/Bangkok	UTC+7
Sydney · Australia	Australia/Sydney	UTC+10

What is the time difference between Bangkok and Sydney?: Bangkok is 3h behind Sydney. This gap is not fixed: daylight saving time moves it between −4h and −3h during the year.
Does the time difference between Bangkok and Sydney change during the year?: Yes. Because of daylight saving time the gap moves between −4h and −3h.
When is the best time to call Sydney from Bangkok?: Between 09:00 and 14:00 in Bangkok, which is 12:00 to 17:00 in Sydney.