I have a sneaking suspicion that an estranged family member may have died.
I am wondering if anyone knows of an agency in Australia (preferably online) where I’d be able to find out?
Apologies for the morbid question 
Annie! Annie! are you OK?
Oh, I guess that’s not what you meant.
That site uses the Social Security Death Index, which is for anyone with a social security number in the USA.
One thing though it’s not 100%. They screwed up the death date on my mother by 6 years. And my father isn’t even listed. Both died in the 70s, so that may be a reason why the dates are wrong or don’t appear
I’ve noticed other things like places of death etc are not always right either. And it took 2 years for them to list someone I know died before it showed up online on the SS death index.
So it can be a useful tool, it’s not 100%
Do you know which state they were in?
Here’s the NSW Registry of Births, Deaths and Marriages
http://www.bdm.nsw.gov.au/deaths/deaths.htm
It’s going to be a different agency for each state and territory.
Which state, and how long ago?
I haven’t researched a lot outside Victoria so I’m not going to say anything about the availability of records in other states, but in this one death records are closed post-1985 for privacy reasons. I can look up the pre-1985 Victorian death records for you if you’d like, or you can pay 99 cents per screen and search the BDM index yourself at this site.
If you can come up with some date/place estimates, you might be able to pay a researcher to hit the newspaper archives at the appropriate State Library. Or you could try newsgroups (ex. soc.genealogy.aus+nz) and see if someone is able to look up newspaper obituaries, or you might find someone local to your relative who is willing to scope out the cemetery.
You could also try the White Pages and see if they still have a phone number listed.
Feel free to PM me if you’d like me to poke around and see what I can find in the records that are available to me.
Last known communication was around July this year. After that, nada. The gentleman in question is from South Australia.
I have found a lot of the pay per view stuff, but I’m not entirely sure how reliable that is. I will probably contact BDM SA soon to find out if he’s still around.
obits.com.au lists all the deaths that use funeral directors since 2006.the ryerson index is good too.
Try shaking them real hard. Or maybe a bucket of water?