Positive: Finally, finally dates of the trip have been determined. Adelaide-Geraldton (via Perth) round trip tickets have been booked and paid for. The plan (still little a bit rough) is:
Depart from Adelaide on April 13th in the morning, stat overnight at Geraldton, and trip to Abrolhos Islands OC-211 by a boat (or plane, still not clear) from Geraldton - April 14th in the morning.
Accordingly I plan to get on air on 14th in the evening (the time of day is local, GMT +8).