Giving someone your power of attorney means that person can do generally any legal activity on your behalf.
operating your bank accounts,
buy and sell real estate (if the POA document is registered with NSW Land Registry Service),
manage investments, and
sign legal documents.
Generally, such power of attorney becomes ineffective if the principal loses capacity, unless the document is an enduring power of attorney – in that the document endures the principal’s loss of capacity.
An enduring power of attorney lets that person conduct that legal activity on your behalf after you have lost capacity – that is, the mental ability to make and understand your own decisions.
Should you not have capacity, then you would not be able to make a valid power of attorney, as you would be deemed to not understand what you are doing by making such document.
It would be too late – you would have missed your opportunity.