Does the time difference between Auckland and Berlin change during the year?

Yes. Because of daylight saving time the gap moves between +10h and +12h.

When is the best time to call Berlin from Auckland?

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

Auckland is 10h ahead of Berlin. This gap is not fixed: daylight saving time moves it between +10h and +12h during the year.

Auckland

New Zealand

--:--:--

Berlin

Germany

--:--:--

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

City	Time zone	UTC offset
Auckland · New Zealand	Pacific/Auckland	UTC+12
Berlin · Germany	Europe/Berlin	UTC+2

What is the time difference between Auckland and Berlin?: Auckland is 10h ahead of Berlin. This gap is not fixed: daylight saving time moves it between +10h and +12h during the year.
Does the time difference between Auckland and Berlin change during the year?: Yes. Because of daylight saving time the gap moves between +10h and +12h.
When is the best time to call Berlin from Auckland?: There is no overlap between standard working days, so one side has to call early or late.