I would write a polite letter to “The owner of lot X” explaining the situation and request that your executive committee arrange for it to be forwarded to the owner of the unit that is using the storage locker that you now understand to be yours. You would ask them to remove their stuff with a reasonable time-frame so that you can use your locker yourself.
If the committee does not co-operate by forwarding your letter, then you can 1) make a fuss about inconsistent treatment etc., and 2) go to the managing agent and ask them to forward the letter yourself, and 3) if that doesn’t work, demand to inspect the unit roll and get the contact details yourself. If the managing agent refuses on (spurious) privacy grounds, point out that you tried to avoid that by asking to have your letter forwarded.
If none of that works, your situation is that somebody has left their property on your property. In the ACT this is covered by the ‘Uncollected Goods Act’. Other jurisdictions would have something similar. As I understand it, you can’t just toss out their stuff. There are provisions about how much effort you have to go to to try to find the owner before you can dispose of the stuff. The level of required effort varies according to the value of the goods. If the goods are high value and you dispose of them by sale, you have to keep the proceeds for a certain amount of time but you can deduct reasonable costs. You could enlist the help of the managing agent for this. They should be accustomed to dealing with this sort of problem. People leave stuff all the time eg. tenants moving out etc. If you act on their advice you would have a reasonable defence against someone accusing you of getting it wrong.