As this is Victoria, it should be noted that committees have considerably more power than some other states. If the works only affected common property in a minor way, the committee might have decided that a full OC vote was unnecessary.
I’m reading (because the OP was not informed of the works) that it was a committee only decision. If so, there should be a record of the decision in the committee minutes. In Victoria, it’s not compulsory to distribute those minutes to each owner, but they should be in the OC records.
If only one lot benefits from the system, that lot automatically must maintain it, regardless of its physical location on common or lot property. That’s part of the OC Act in Victoria.
If it was done by committee decision only, it was poor form of the committee IMO. While it’s quite common to allow lot equipment to be installed on common property, decisions should be fair to all owners.
What has been decided by committee only, can be altered by a future committee, or by the OC at a general meeting or of course by VCAT. So there are some possible options to investigate.