I have conducted hundreds of simple legal cheap ceremonies for couples since I became a Marriage Celebrant in 2004. Either at my Home Office or at any venue of your choosing (including an airport lounge as you are about to fly out of Australia).
I realised there were many couples who wanted simply to obtain the Certificate of Marriage that was registered in Australia but not have the large ceremony in front of family and friends at that time.
As you know almost all marriages have two parts to it. The first is the legal part where you become legally married. The second part is where the couple celebrate the ceremony in front of family and friends. A good percentage of marriages include the legal part and the ceremonial part together and it all happens together on the same day.
If you just want to get the certificate with just two witnesses and without more than 4 other guests I can offer this very cheap solution. From $360 to have the ceremony at my home office. I have the signing table and chairs but don’t have a photographer usually. However one can be arranged for a small fee.