I has a similar issue once in our block. The (non -resident) owner used the garage to store chemicals, paints, paper, bags rubbish etc for his business. We are caged, but it was an eyesore, the tenants had no where to park, and was a potential fire hazard. Check your By-Laws. Ours had a standard clause regarding “Storage of inflammable materials”. A letter from the strata manager solved the problem. They would also be in breech of the Visitor parking bylaws as well. So could also send breech notice for that also. Take photos, document everything and make the Strata Manager do all the work to remove yourself somewhat from it being a personal issue. Hopefully the risk of a $2000+ fine will be enough to make them do the right thing, But be prepared that 3x breech notices will have to go out first before you are set to escalate to NCAT for enforcement and fines (s0 I was once advised).
Keep in mind the dividing fences act also. If you paying for it, and its on your side of the dividing line, then you don’t need the other parties approval. Just make sure its professionally installed (doesn’t present a hazard); is in keeping with building (same as what is already installed); and doesn’t affect common property (lighting etc)- to keep strata happy. I’m not an expert in this so by all means get your own advice. -R