The lot owner has been breached multiple times and has received a notice to comply.
and
…we’ve been told mediation is the next step and after that NCAT tribunal which could cost upwards of $20K.
Sounds to me like you are getting bad advice. The resident has not been “breached” until they receive a Notice to Comply. I hope you haven’t fallen for the nonsense that you need to send three warning letters before you can issue an NTC. If the owner has been sent a Notice To Comply, mediation is not required: you go straight to NCAT and seek a fine if the breach notice is ignored. Also, if they defend the NTC when it is clear that they were in breach and knew they were, you have a fairly reasonable chance of having costs awarded.
Whoever is giving you advice is either not giving you the whole picture or isn’t explaining themselves well.