JC – as the problem only occurs in some areas of the Plan it’s not likely to be with Sydney Water’s supply, but what did the Owners Corporation’s plumber report? How old is the building? Is its layout such that different water supply lines supply different areas of that building? Can any of those be isolated? Are units separately metered?
Sorry for all the questions, but they’re some of the ones that your Strata Manager should be answering on behalf of your Owners Corporation (O/C), because it’s the O/C that’s responsible for investigating and rectifying the problem, and depending upon the layout of the internal water supply, some strategically located in-line “dirt boxes” could both identify the source/s of the grit and capture whatever’s circulating within the system.
How to handle it?
Have all similarly affected Owners that you can contact sign a letter to the Secretary of your Executive Committee (or to the Strata Manager if they’re delegated that responsibility) explaining the problem/s as succinctly as you collectively can, and then requiring the O/C to within say 7 days and under the provisions of Sect 62 of the NSW Strata Schemes Management Act (1996), do all things necessary to properly maintain all its common property water supply network to both identify the source of the grit problem, and if it’s found to be associated with that network, to rectify the problem.
If your Executive Committee doesn’t act or respond with the suggested timeframe, then come back here for further advice about how to then force the issue.