If you know who the landlord is, in NSW you can possibly use some of the provisions HERE. Otherwise NSW does have an Uncollected Goods Act but both pieces of legislation put the onus on the holder of the goods to notify the person who has left them.
I would think if this was a recurring problem, it might be worth drafting a by-law that said goods left on common property for more than, say, 14 or 30 days would be deemed abandoned and would be subject to disposal by the the Owners Corp by whatever means they saw fit. The by-law could specify areas that were included – like a storeroom – and conditions under which things could be left there, including written permission for a specific period and charges for storage.
It could also state that abandoned goods might be sold with the cost of disposal subtracted from the sum received. This could probably be retrospective in that it would apply to goods still in common property when the by-law came into effect. It would be worth talking to a lawyer about this.