Why would tax outweigh the value of the property? It's possible only if your grandma has some debts and then you would inherit debts with the apartment. And why are you afraid that your grandma will die before she passes you her flat in inheritance. To do that she has to simply write a will witnessed by a notary. Notary can even come to her house.
Moreover in Poland closest family don't pay any tax from inheritance. Grandson is considered close family:
The law on inheritance tax and gift exempts entirely from tax the acquisition of property or property rights by members of the so-called zero-tax group. This is a spouse, descendants (children, grandchildren, great-grandchildren ...), but also (parents, grandparents, great-grandparents ..), stepchild, siblings, stepfather and stepmother. Adoptive family also benefits from discount.
These individuals do not pay tax neither from inheritance or from donation - but only if they notify the fact of the acquisition of the assets to the competent tax office. For make such a statement, the entitled to relief generally have six months from the date of the tax obligation. So either 6 months from the validation of the court's decision on the purchase of the estate, or 6 months from the date of registration of a notary act of succession certification.
But because you're not resident of Poland, probably you will have to pay inheritance tax according to Australian law.