There’s an old saying that “defamation is for people with deep pockets” – and that applies to both sides. Unless you have a shed-load of money lying around, you can’t even think about hiring the kind of lawyers you’d need to effectively pursue a defamation case. And even if you win, there’s nothing to stop the other side declaring bankruptcy and leaving you to pay your own costs.
A more fruitful route may be to seek an Apprehended Personal Violence Order (APVO) through your local court. APVOs cover intimidation, harassment and stalking. If you think you can prove any or all of these things, then this would be a much less expensive and possibly more effective way of dealing with this.
In the meantime you could write a letter to your committee and insist that they circulate it to all residents, refuting the claims in the previous letter, detailing the harassment you have endured and pointing out that if it doesn’t cease immediately, and if they don’t circulate your letter, you will be seeking an APVO.
And you should tell your landlord that this is what you plan to do. As members of the owners corporation, and your landlord, they have a duty to protect you from this kind of treatment. But you want to have them onside, so I wouldn’t be too aggressive.
I’m no lawyer but I believe verbal assaults and “poison pen” letters come under the definitions of “violence” as far as APVOs are concerned. That could be your best avenue for resolving this issue.