No reasonable person could suggest that you haven’t done everything possible to appease your complaining Owner/EC Member.
Check the Consent that you originally received to install the air conditioner, and look for any conditions relating to times of operation and noise.
If your tenants are complying with those conditions and the normal requirements of the Protection of the Environment Operations Regulation (2008) where operation of air conditioners is generally permitted only between 7am-10pm Mon to Sat, and between 8am-10pm Sun and Public Holidays, then I’d do nothing other than to ignore the complaints.
However if you want to take the initiative, apply to the NSW Office of Fair Trading for Mediation of your dispute by completing and lodging this Form https://www.fairtrading.nsw.gov.au/pdfs/About_us/Forms/medform.pdf