What legal document is needed for a child to manage an elderly parent's finances in Australia?

They need to have already made an enduring power of attorney covering financial matters. This document allows the nominated attorney to then step in, access their bank accounts, sell their property if needed, and otherwise manage their financial affairs. But remember, the document has to be signed and witnessed whilst the person has capacity to understand the nature and effect of the document. If they don't have capacity, it's too late and an application may need to be made to a state tribunal for an order appointing an administrator.