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

Yes. Because of daylight saving time the gap moves between +8h and +10h.

When is the best time to call Zurich from Sydney?

There is no overlap between standard working days, so one side has to call early or late.

Sydney is 8h ahead of Zurich. This gap is not fixed: daylight saving time moves it between +8h and +10h during the year.

Sydney

Australia

--:--:--

Zurich

Switzerland

--:--:--

A 9-to-5 working day in Sydney does not overlap one in Zurich at all — somebody has to take the call outside office hours.

City	Time zone	UTC offset
Sydney · Australia	Australia/Sydney	UTC+10
Zurich · Switzerland	Europe/Zurich	UTC+2

What is the time difference between Sydney and Zurich?: Sydney is 8h ahead of Zurich. This gap is not fixed: daylight saving time moves it between +8h and +10h during the year.
Does the time difference between Sydney and Zurich change during the year?: Yes. Because of daylight saving time the gap moves between +8h and +10h.
When is the best time to call Zurich from Sydney?: There is no overlap between standard working days, so one side has to call early or late.