The name change was prior to her abruptly leaving her job and selling her home. She had time to change her name and apply for a passport in that name. If, as Florabella, she was able to obtain citizenship of a European country, she could then change her name again, and completely disappear. In Australia, she only changed her name on her passport- everything else was in Marion Barter. My theory- she changed her name on her passport so she could come back into the country without being traced, liquidate her assets, and then move on with whatever the rest of the plan was. Did she plan to disappear forever? No idea- but whatever the plan was, it kept being ruined. Sally, through no fault of her own, busted her mother with mystery man. Sally found out she was back in the country. The security guard at Ashmore, where she lived for a number of years (where she was not a tourist, but a local), was the one who identified her, who had the message "tell Sally I'm angry about the car". Marion's attempts to do things under the radar kept getting stuck. Why the $5000 a day? Intriguedbyintrigue mentioned the FTRA- Financial Transaction Reporting Act.
Marion was trying to stay unobserved, and would have for a long time, had the bank not revealed what was going on in Byron.