I've asked my mum and she said I've never had a PESEL. The passports were child passports, I still have one (expired 2011).
I checked again, and it looks like it's possible to get one without a PESEL. In your case, with a passport that expired in 2011, you won't have any difficulties.
The website is here - sydney.msz.gov.pl/pl/informacje_konsularne/paszporty/paszporty
- you should just apply for a new one. You don't need to worry about claiming citizenship from your parents or grandparents, as you've already had a passport issued in your own name.
It should be pretty straightforward, but the lack of PESEL might cause issues - I'd call the consulate in Sydney and double check.