Really, if your Executive Committee and/or strata manager can’t take this on, they are not much cop. This is such a clear breach of even the most basic by-laws that a simple Notice To Comply should be issued at the earliest opportunity – but they won’t do that if you don’t complain in writing. Get your neighbour who also suffers to back you up .
Your next step, if they fail to get on board, is to apply to Fair Trading (tel 13 32 20) for mediation (which will cost $76 dollars or thereabouts) and then the CTTT if that doesn’t work. You can’t skip the mediation, by the way.
As Whale says, it’s probably just a blocked drain (unless they are blithely hosing the dirt off their balustrade on to your deck). But, to reiterate, this is an issue that the EC should take on and I can’t think of any reason why you wouldn’t go to them with this problem.