Sharon,
I am puzzled about the purpose of your mission. Are you just concerned about the stones rotting away or being vandalised ? Hoping to attract astronomically-minded visitors to the park to see them ? Upset because people can't see them ?
Or is this supposed to be a memorial ?
There already is a memorial (the modern obelisk) and neither it nor the remaining stones hold any interest to the astronomically-minded.
So if you are concerned about the stones, call the Powerhouse Museum or Sydney observatory. The curators at the museum would have the best idea what to do (take them out of the weather and put them in storage).
As for history of Parramatta observatory there is a summary here, with a list of references:
http://www.southastrodel.com/Page032.htm
Unless you have the observatory building (or a replica) AND displaying the actual telescopes, eyepieces and micrometers etc, and hopefully some of the records (diaries, observation logs etc) then quite frankly no-one will be interested.
This is what is wrong with Greenwich - in 1975 and 1989 it was a fascinating place to visit, but now that they have removed almost all the telescopes and astronomical instruments it holds almost no interest to me now, and was utterly uninformative for my wife. A shop selling replica trinkets for idiots, a powerpoint slide show and a tawdry plastic GOTO planetarium was of no use whatever.