I had a couple of phone calls from the real estate agent who got us into this house yesterday. He made vague threats about my web page, which details our troubles with them.
Fortunately the defamation law in NSW has changed in the last couple of years to be less corporation/rich/powerful-friendly. You no longer have to prove "truth and public interest" as a defence, merely "truth". As all I've done is report the truth, and give my own opinion, I don't see any problem.
We're lucky that the lease is actually between us and the landlord directly, so we don't have to deal with this company any more. The landlord himself is a lovely bloke and lives just up the road.