Dueling experts! But the devil is in the detail. Reading between the lines, which may be me misunderstanding thing, I am guessing that what may have happened is this: You had water ingress in 2023, and an expert produced a report indicating that the cause was a number of problems. The Owners Corporation address some (or most? or few?) of them, fixing the leaking downpipes and gutters. Now in 2026 you have (ongoing, new, different, the same) water ingress problems. The Owners Corporation has indicated that it believes there are no current problems, and if you think so, they you need to present an engineer’s report, and pay for that, proving this…
I think the Owners Corporation has a point that a fresh review of the problem is appropriate, as the original issue was fixed at least to some extent, and 3 years have elapsed. If you had ongoing issues during the last 3 years, you missed an opportunity to make the OC aware of this (I’m guessing). Your current problems may be a recurrence of the original issues (ie, the gutters are blocked again?).
But if you have a water ingress problem, and you have reasonable evidence about it, it is the Owners Corporation’s responsibility to investigate and fix it. They should arrange an engineer or expert to investigate and report. It’s not like owners are rationed to share out problems and fixes.
You might have an attachment to the original engineers report and their recommendations. That should not matter, and in fact, having the Owners Corporation getting a new report from a new expert is better, and that person will have the original report to reference. My advice is not to re-litigate the past problem, but to focus on the current one, and insist it be dealt with. Don’t try to tell the OC how to go about this, and to insist on a specific expert being involved, but focus on the current problem (dampness, mould, buckets of water, photos of trickles etc) and just keep asking the OC to fix it.
If they take the view, that – “hey stop being greedy, we already fixed a problem in your Unit a couple of years ago, wait your turn” then go to NCAT – or offer to do so, and prepare to get legal advice and support.