You must have had documentation somewhere for the pr application. Even an expired passport should work. Do your parents not have any records, even digital ones from that time?
Do you have a Medicare card even?
I don’t think a migration agent can help unless one was used for the pr application and still holds records.
I would seek help from your local MP they should have an office locally and may be able to point you in the right direction or intercede on your behalf. I’m sure you are not the only one in this situation, for example many refugees arrive with nothing!