Not sure if this helps, or you already had this information, but Sveta Petka would be the village Agia Paraskevi today in the Florina (Lerin) prefecture (https://en.wikipedia.org/wiki/Agia_Paraskevi,_Florina
As a direct relative, you should be able to request a Family Status Certificate (Pistopoiitikó Oikogeneiakís Katástasis) from the Greek Consulate in Melbourne from your father and grandfather. Typically they ask for name, parents' names and date of birth for the person whose Family Status Certificate you're requesting.
If you don't have the date of birth and name of your grandfather's parents, you could try to get your father's Certificate instead, which would list the full names of his parents (i.e. your grandfather) and I *think* dates of birth (I can't recall exactly how the information is presented on the certificates) of his parents.
Stick to the newer Greek names when corresponding with the Consulate if you want a smooth experience...speaking from experience ;)