We have a similar problem. Our building in NSW unfortunately has planter boxes on each balcony. Some people keep them clean, some have pot plants in them, and some allow a buildup of leaves and weeds in them. It is difficult to inspect them as this requires access through each individual unit.
If a resident allows their planter box to fill up to such an extent that it impedes water drainage off the balcony and so causes water damage to their unit, is it their responsibility or the owners committee for not cleaning them? We, the committee, have previously circulated notes reminding owners to keep their planter boxes clean, but recently our strata manager said we had to clean them because the balconies and planter boxes are common property. From what Jimmy says I take it as the owners responsibility, but your confirmation/advice would be much appreciated.