Have you had a look at the actual easement document lodged by the Council in respect of the land? It would be held by the Land Titles Registration Office in your State.
Once you have a copy of that, you can read it to determine what the original easement was that was agreed between the parties in the 1980s and the Council, if there was any agreement at all.
If the easement has been created, however, not by the lodgement of a certificate of easement but by some other legislation creating the easement, you may have a better chance.
The reason for this is that under the Real Property Act 1886 (SA) section 86, if the public (i.e. the public via its Council) has an easement IN the land, (as opposed to over it or on it or along it or across it) then you have no rights to alter that public use: http://www5.austlii.edu.au/au/legis/sa/consol_act/rpa1886178/s86.html
So the only solution I can think of is if the easement itself as registered with the Land Titles Registration office refers to some other legislation that gives the easement its validity, we could then search that legislation to see whether it overrides this provision or conflicts with it, in order to argue your case.
If you would like further service, namely, I could search the title to the property to unveil any registered easement or reference to other legislation, I will offer you additional service so you can get me to do that and advise you further.
I hope that has helped you today.