I went to Optus today to change/update my last as I have an online order and they cannot process it because of my surname..I went there and show them my driver's license but the lady there said that I need to bring/show my marriage certificate as well and I don't have it as I got married overseas...what can I do now? TIA.
Its hard to know how obstinate Optus would be.
Do you have enough ID in your new name to provide identification? This is the list.
The marriage certificate is only needed if all your other documents are in your old name (If so how is the new order in a new name?).
You might consider going the full rename process. Read the Married Overseas section here.