Your plan seems OK.
I suggest that you bring in an a/c installer and get them to take a look at what remedies are available to properly drain the unit, and after the assessment I would ask that the a/c professional writes a report including photographs etc. If you end up taking this to NCAT then you will need to also submit this professional report.
Is the OC OK with the location of the exterior unit? Is it located on a common property wall or within the owner’s own Lot? Did the pipe work need to go through a common property wall, or a window? Is the issue only with the drainage? What do your by-laws say about a/c units? Is the a/c in breach of the ‘appearance of a Lot’ by-laws?
Your OC’s future actions depend on the answers to the questions in the above paragraph.
Below is an article on the correct drainage requirements for an a/c. Although it is a Victorian article the plumbing requirements should apply Australia wide:
NB: I have had a blocked a/c drainage pipe which was caused by mud dauber wasps building a mud nest in the outlet. It was an easy fix with a metal skewer used to dig out the wasp and some fly wire taped to the end of the pipe to keep the wasps out!! Perhaps the new owner’s a/c leaking problems may also have an easy fix.