Hi Zoe, I have dealt with a similar situation in Tasmania, and it has ended up being reported to the Police Fraud Squad for investigation and possible criminal prosecution for fraud.. in that case the parent had dementia, and had no idea what was going on with the money.
The most difficult thing in these situations is normally obtaining the required evidence of the misappropriation of funds.
As you were also Power of Attorney with direct access to all records, bank accounts, etc. I assume obtaining the evidence will be easy for you.
Do you already possess the required evidence, or will need assistance to obtain more?
Did your late mother suffer from any mental disability? dementia? that meant she would not understand what was happening with her financial affairs? Is it possible she gifted the money to her grandson? or knew and approved of the expenditure?
In NSW... would need to look at the Powers of Attorney legislation, and determine civil breaches and remedies... as the grandson now holds real estate that should theoretically be the property of your late mother's estate and be available for distribution to the beneficiaries of her Will.
Kind regards