Cate – it appears that as in NSW your Strata Company is required to provide intending purchasers (or their lawyers / property conveyancers) with certain prescribed information in the form of a certificate, which amongst other things must incorporate the minutes of Strata Company and Council meetings and “any other record or document in the custody or under the control of the strata company”.
It seems to me that it would be prudent for your Strata Company or Council to meet, and to then minute something referencing the unauthorised works that the Owner has undertaken and to provide that Owner with a copy, or alternatively at least correspond with that Owner pointing out the unauthorised works so that a “record” of that could be disclosed under Sect 43 of the WA Strata Titles Act, and as Peter C suggested (post #3), perhaps the existence of that record could be discretely brought to the attention of the Selling Agent.