G’day Jimmy, I looked at this one last night and whilst I instantly jumped to an opinion, circumstances resulted in me having a bit more time to think it over.
So…whilst I don’t agree with the presumption that simply everything that’s in the Common Property is Common Property, particularly if the item is not available for or to the benefit of the common (everyone), until there’s a more definitive “who’s responsible” document available or some revised Legislation that’s less open to interpretation, I agree with you that the transformers and even the downlight housings themselves are the Owners Corporation’s (O/C) responsibility.
My only qualification would be that those items were installed in the Lot as part of the original construction of the Plan.
So far as the replacements are concerned, suggest that for an equivalent cost your O/C could replace the transformers with a 240VAC adaptor and matched halogen sealed-beam downlight, or if white light is preferable to a bright light then a sealed-beam LED is an even better and much cooler option.