In that case, I see two options you could create a written agreement with the dates of the original agreement - since you would only be formalising an agreement that has already been made this would not be deceptive or misleading.
I would be cautious however not to have your parents investigated by the ATO etc. Since they have made a profit from the loan it would (as far as I am aware) be considered income. If it were considered income by the ATO I would assume they would also need an Australian Credit Licence or similar. I am no expert on the matter but just some food for thought.
It might be an idea to consider the money your parents gave you as payment for some "service" which you were unable to perform and have now incurred a debt i.e. a refund of the amount borrowed.
