Agree with Carole. You might need to find out if there is anyone buried in the same grave. Even if there is you can probably still erect a headstone. The cemetery should be able to guide you on this.
Depending on whereabouts in Australia the grave is and what records are available, there is a lot online these days, and some records do show other internments. This is the case for some of my ancestors are were buried in a grave with an earlier burial of an unrelated child. If my memory serves me correctly, they do have a headstone, which names the couple, but not the other resident.
Regarding informing other descendants, unless they are interested in family history they may not even be aware of this couple, and tracking down possibly hundreds of living descendants in Australia may be an impossible task. If you have names, electoral rolls might be a place to look, though I think they only go up to 1981? Then there is social media of course.