Does the time difference between Hong Kong and Sydney change during the year?

Yes. Because of daylight saving time the gap moves between −3h and −2h.

When is the best time to call Sydney from Hong Kong?

Between 09:00 and 15:00 in Hong Kong, which is 11:00 to 17:00 in Sydney.

Hong Kong is 2h behind Sydney. This gap is not fixed: daylight saving time moves it between −3h and −2h during the year.

Hong Kong

Hong Kong SAR China

--:--:--

Sydney

Australia

--:--:--

A 9-to-5 day in each city overlaps for 6h. That is 09:00–15:00 in Hong Kong, which is 11:00–17:00 in Sydney.

City	Time zone	UTC offset
Hong Kong · Hong Kong SAR China	Asia/Hong_Kong	UTC+8
Sydney · Australia	Australia/Sydney	UTC+10

What is the time difference between Hong Kong and Sydney?: Hong Kong is 2h behind Sydney. This gap is not fixed: daylight saving time moves it between −3h and −2h during the year.
Does the time difference between Hong Kong and Sydney change during the year?: Yes. Because of daylight saving time the gap moves between −3h and −2h.
When is the best time to call Sydney from Hong Kong?: Between 09:00 and 15:00 in Hong Kong, which is 11:00 to 17:00 in Sydney.