Jeggers, my sympathies to you. Regarding the concreting, if you are an owner, there’s nothing to stop you from obtaining the requisite quotes and forwarding these to the strata manager. Of course it’s their job to do this, but if you need it done quickly and the agent is useless, why not? It would also shame them and you could use this as an example to other owners as to why you need a new strata manager.
Regarding the problem tenants, under common law you have a lawful right to feel safe, free from violence and harassment, and to be able to enjoy your property in peace and quiet. In other words, you can sue. But you’ll need a lawyer and you need to balance the risks of failing the legal thresholds, and the risk of deadbeats not paying up, even if you win. For a legislative approach, under s153 of the Strata Schemes Management Act NSW, owners, occupiers and other persons are not to create nuisance. Other states may have similar laws. In NSW you could apply directly to NCAT to enforce s153. Make sure you diarise and document everything. Dates, times, who and what, whether you felt fear as a result of acts, recordings, photos, videos, etc.
You could also take the criminal approach. Again, you have to diarise and document everything in detail as above. You may need to install video cameras on your property running 24/7. Have the video on your phone running whenever you feel unsafe or threatened but don’t use it to intimidate the perpetrators. You are collecting the evidence to protect your lawful interests, not to infringe other’s interests. Familiarise yourself with the Crimes (Domestic and Personal Violence) Act (NSW) or similar legislation in other states. Note s13 of the Act. You could also apply to the local court for an APVO to protect you from violence if you have evidence.
Also familiarise yourself with the Affray and Assault provisions of the NSW Crimes Act. Once you have solid evidence, draft a statement of what happened, whether you were afraid of violence against you, and go to the police. The police may and try and fob you off, but if an offence has been committed and you have evidence, they must take your statement.
Good luck.