I'm trying to gain EU citizenship through my father who is of Greek heritage. My grandparents were born in Greece and moved to Australia and got married here in a Greek Orthodox church. My father is an Australian citizen, as is my mother. They were married in a religious ceremony in a Greek orthodox church in 1986. My father was baptised in a Greek church, as was I. My father moved back to Greece for a few years where my Uncle was born and he attended a few years of primary school before moving back to Australia. I still have family living in Greece.
Am I eligible to apply for dual citizenship? Given the fact that my grandparents were married in Australia could be a problem?

You very likely have a claim to Greek citizenship through your father.

If your father's parents were Greek citizens [you mention they were born in Greece, but you would have to confirm they were Greek citizens from the municipality in Greece where they were registered], were married in the Orthodox Church before 1982, and gave birth to your father in wedlock, then your father has a claim to Greek citizenship from his birth.

Since your parents were married after 1982, it is irrelevant whether they were married in a Church or not, as Greece recognizes all marriages after 1982. If you were born in wedlock to a Greek citizen father [which it sounds like you are], then you are also a Greek citizen by birth.

All you have to establish is the Greek citizenship of your grandparents. Once that is verified, your local Greek consulate can help you establish your claim to citizenship as well. You will need birth/wedding certificates etc [including the Orthodox wedding certificate of your grandparents] but your case sounds pretty straightforward.

Best of luck!

Thanks for your reply. I've been in touch with the consulate and the biggest issue at the moment is whether my grandparents registered their marriage in Greece. Although they were born in Greece and had Greek citizenship this is apparently not enough and we need to show that they registered their marriage. According to my grandmother she paid the minister at the church the fee to get their marriage registered but never got any sort of documentation to prove it.
Aside from going to Greece is there any way I can find out if their marriage was registered?

The minister [priest, actually] at the Church would not have registered the wedding at their municipality in Greece - he would have only registered it with the relevant diocese.

Still, given that you have proven both your grandparents were Greek citizens, and were married in the Orthodox Church before 1982, you should be able to make your case. I assume your parents were married in Australia in a Greek Orthodox Church. In a perfect world, you will actually have their Orthodox Wedding Certificate [Πιστοποιητικο Στεψης] or equivalent which includes the signature and seal of the relevant Bishop in Australia. If you do not have that, you can go to the Church and have them write you up a "ΒΕΒΑΙΩΣΗ" with all the relevant details that already exist in the Church's records. This document will then have to be signed/sealed by the bishop, and then the bishop's signature will have to be certified by the relevant office in the Church of Greece [Μονή Πετράκη]. There may be one last step involving the Court of First Instance in Athens, but then you should be able to register that wedding in the Special Registry in Athens - allowing your father to claim his citizenship, and then you to claim yours.

It sounds convoluted, but you should be able to do a lot of it through one of your relatives in Greece that you mentioned... Best of luck!

Thank you for that! So far we've gotten my parent's marriage certificate from the church that they were married in here in Aus but haven't done anything about getting their marriage registered in Greece yet. Thanks for the advice :) very helpful.
Hopefully it will work out in the end!