Adelaide is 10.5 hours ahead of Reykjavik
There is no standard 8-to-6 overlap. Someone joins early or late.
The 10.5-hour offset between Reykjavik and Adelaide means there's no point hunting for a 9-to-5 overlap — there isn't one. Expect compromise: someone in Reykjavik or Adelaide will need to take the call outside the usual 9-to-5, so share that load. Finally, Adelaide observes daylight saving and Reykjavik doesn't, so this window shifts by an hour for part of the year — recheck it each spring and autumn.