Hi Yasmina,
You can still attend the Open Day in Melbourne on the 22nd. You won't be marked absent for the cities they invited you to, because none of them are Assessment Days - they are all OPEN DAYS. Only AD's do they invite specific candidates and mark whether or not you attend. I suggest you attend the one in Melbourne if going to Newcastle is too expensive. I'm not sure why the Melbourne OD didn't come up on the invite they sent you. But it doesn't mean you can't attend. Hope this helps
PS. I might see you there