Portland is 18.5 hours behind Adelaide
Best overlap: 8:00 AM to 1:00 PM in Adelaide is 1:30 PM to 6:30 PM in Portland.
Schedule calls for 8:00 AM to 1:00 PM in Adelaide — your colleagues in Portland will be online from 1:30 PM to 6:30 PM. Around 5 hours of overlap is enough to work comfortably, as long as meetings don't drift to the end of one team's day. One thing to flag — this overlap crosses the date line, so Portland is on the previous day relative to Adelaide.