A few points:
1) The flight to Xmas Island departs from Per international terminal so the liquid rule applies. Technically you only need your Australian Driver Licence as it is part of Australia but it is advisable to carry your passport as Jakarta is only 500 kilometres North of Xmas Island and is an alternative airport for emergencies. Landing at CGK with only your Australian DL is not going to get you very far. All the Va crew have to bring their passports!
2) A car is essential for getting around Xmas Island, preferrably a 4WD as there are many gravel, unsealed roads...we hired a Toyota RAV
3) If you are making the effort to go to Xmas Island, It is worthwhils stopping over at Cocos Keeling Islands as well (Cossies beach on Direction Island was named Australia's best beach in 2017)
One day the VA flights goes Per-Xch-Cck-Per (Tues when I went) and the other day (Sat when i went), it was Per-Cck-Xch-Per)
4) We only stayed 2 nights at Xmas Island because of flight delays ex Cocos but a mininum of 3 nights at Xmas Island should be enough. You really need to work your stay length around the flight schedules esp if you visit both islands.
5) Be prepared for flight delays / cancellations meaning you could be stranded on the islands longer than planned so have adequate travel insurance. Exmouth is also an alternative airport on the mainland for emergencies. Insurance is important for Medical Evacuations to Per as there are limited health care facilities on both islands
6) book accommodation early....When we went in late November all the accommodation in Xmas Island was booked out by bird lovers! (apparently Xmas island is a bird watching paradise!!)