It is my private laptop that I own
Can you prove that you owned it before you moved to Poland? And even if you can, getting temporary clearance for personal possessions is a pain in the arse.
It seems they are trying to put an import tax on it or something
They will be adding 12% duty to the value of the item and the cost of the shipping (I think it's 12% for that item from that location) and then 23% VAT on top of that.
Frankly your best bet is to just tell them to send it back to Australia and then either ask your parents to sell the thing so you can buy one here or to just have it put in the suitcase of the next person you know who is travelling from Oz to the EU (if it's sent from anywhere in the EU there will be no tax to import it to Poland).
Surely this is ridiculous?
Yes, but T.I.P. (this is Poland).